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[Othob Rithol]

Support the Eternal Suggestions System
See also the surface thread, Barbarossa the Seal God's Aboveground Diversity.
This is a brainstorming thread about features, creatures and events underground. Please feel free to contribute your ideas, point out flaws, or provide useful images, links or source materials.
This is not a discussion about game mechanics, balance issues, bugs or anything else not related to features, races, plants or animals under the surface.
UNDER NO CONDITION is this thread meant to be the end all, be all absolute PROPER place to post a UD related suggestion. DO NOT go around berating others for not posting here. Instead invite them here, or link their suggestion into this thread.
"^"s are external definitions and references (wiks, etc).
"*"s next to entries are links to relevant source threads
"(I)"s next to entries refer to features that could have an init option to disable due to FPS, annoyance or difficulty
See Toady's 10/15/08 comments here.
General Suggestions:
The underground world could be so much more diverse, with surprises to be found and opportunities to be exploited. The underground world could be as varied and diverse as the world above ground. Who says cave adaptation is a hindrance?

• Chasms, Cave ponds and  Cave Rivers in Non-Mountainous Biomes: Perhaps less common, but still possible to find if one digs deep enough. Including traditional HFS in this is controversial. there could be crevasses in the glaciers, populated by yeti and other snowy threats.
  
• Large Underground Caverns: Potentially a whole other world underground. Extensive interconnecting caverns spanning multiple Z-levels could link underground features and off map sites. Features could include stalactites and stalagmites ranging from single tile icons to multi-tile structures to be excavated and built from/upon. See also here.
  
• More varied and natural cave, cavern and chasm shapes: Probably a tough one to fill, but it would be nice if features were not cookie-cutter presses with sheer, unchanging sides from top to oblivion below. See excellent suggestion here .
  
• A Reason to Dig Deeper: Another bit of nostalgia from the 2D version. Perhaps the density/frequency of ore and gems could increase as you dig deeper into the mountain. Not overly necessary right now, but with the features described in this list, motivating the player to delve into the depths might be desirable.
  
• Beware the Deeps: * * * (I) Currently only flying rocks fall, making the underground a very safe place. In reality the deep tunnels are very hazardous locations. Earthquakes can open rifts and caverns, collapse rooms, or leave unstable rock. Rock Bursts are an unpredictable threat that still takes the lives of modern miners. Some sections of rock may not actually be able to support the surrounding rock. Excavations in the area may appear safe, but lead to a cave-in. A highly skilled miner could detect unstable formations, and cause them to be highlighted in the dig screen like wet or hot rock.
  
• Deeper and Deeper: * (I) Since we're talking about adding more features to the underground area, we might want to have the map automatically generate and add more z levels when you reach the bottom.

  more underground diversity
  -- up on dev_next in part, though there are some issues with some of the desired ones, especially with digging down, since it can't load all that many layers, and solutions for slowing downward digging all seem sort of gamey or sketchy
  
• Natural Gas Pockets: (I) A real life danger to miners, these pockets could be exposed causing suffocation, or potentially an explosion if they access a fire or other extreme heat source (ie magma). Other trapped gases could cause tantrums, erratic behavior, or all manner of trouble.
  
• Ventilation: (I) Getting fresh air down to the depths could provide a challenge. Implemented as a new flow, or as a simple "distance to the nearest air shaft", a lack of fresh air could slow and tire a worker. In extreme cases it might even lead to suffocation. Ventilation shafts could be augmented by mechanical fans.
  
• Black Gold!: It might be possible to find pockets, or even an "aquifer" of oil. Whereas the use of petroleum fuel is not really appropriate to DF technology wise, it could provide an added challenge, use as a lubricant, material for extracts, as well as interesting oil/magma traps. This could also serve as a home biome for petroleum based creatures.
  
• Empty Cave Rivers, Ponds and Magma Features: We already see some of this in "pit" chasms, but any underground feature might be empty. But it may not be empty in that you might find a cave forest, civilization, or creature that has adopted the space.
  
• Collapsing Dirt: With the addition of burrowing creatures the need to eliminate the tunnels becomes necessary, else the map will be swiss cheese very quickly. It has been suggested that excavations in dirt slowly refill or become impassable (perhaps one tile from every wall each season?). When it is important that a dirt tunnel or chamber remain intact (ie farms) then it is necessary to buttress back the dirt with rock or wooden walls.
  
• Realistic Rock Hardness: ^ Many players think that mining is too fast and easy. One solution is to have rocks have a a set hardness like Moh's Scale or better yet, Size Strength Metrics. Harder rock could require some combination of extra time, higher mining skill or pick material/quality to dig.
  
• Geodes and Crystal Caves: ^ ^ ^ Real life geodes can be as large as mini-vans. In DF they could easily be the size of a current cave pond or magma pool. The interior crystals could be just about any gem in the game or even flux stone. Glass lovers would be thrilled to discover a geode containing 50+ rock crystals.
  
• Hot Geysers: Not found in the Magma or Water sections because it involves both. Water close enough to magma could be heated and potentially produce steam. This can be utilized for a variety of purposes. Traps, Steam Power (if Toady goes there), steam baths, and ruining your FPS from flows are all possible. Also poses a new danger to miners if you accidentally open a natural geyser.
  
• 3-D Ore Veins: Those of us (un)lucky enough to have played with HFS know that not all veins are strictly two dimensional. Much like real life ore veins could span multiple z-levels.

• Folded Rock Strata: ^ Although it would likely be difficult to implement, folded structures would add interesting details to the surface layers.
  
• Stairs in Forts, Ramps in Caves: * When the game generates a fortress in adventurer mode, instead of using ramps, it should use stairs. Likewise, in a natural caves ramps should be used instead of stairs.
  
• Underground Features Affecting Surface Biomes: Some underground features (notably HFS) could have an effect on the surface savagery levels.
  
• The Basis of the Dwarven Faith: Buried deep underground, and exceptionally rare, the halls of the dwarven gods could be found, as detail in this post.
  
• Crystal Farms: Certain types of crystals could slowly grow on the surrounding rock, and could be mined out slowly to have a steady supply of gems or for crystal glass. Similar to the "Living Rock" and "Cave Kudzu" entries, the value of this feature could be balanced with dangers. One suggested addition includes spontaneous growth from contact with miasma and/or remains. Discussion begins here.
  
• Heart of the Mountain: Buried deep in the core of the highest peaks is a rare crystal formation that is somehow tied to the very health of the mountain. While highly valuable, mining these crystals could have odd and potentially dangerous effects. Discussion begins here.
  
• Salt Deposits: A new type of economic (rock salt) with export value. It can be processed into an extract (salt) that is usable as a spice. For completeness, forts with access to salt water could also produce this by boiling down the sea water.
  
• Impact Craters: A rather rare but valuable find is the site of an impact from a fallen star. Not only is a crater (maybe with a lake) an awesome location to build in, the possibility of finding iron, nickel, or some sort of star metal makes this a sought after feature. Discussion begins here.
  
• Black Lung: ^The bane of real world coal miners, our little dwarves might pay very badly for the sweet, sweet steel. Coal Dust + Lungs = Bad.
  

Remnants of Previous Excavations
In the long history of the world it is more than possible that a fortress or adventurer might find an excavation made by a previous group.... lucky or tragic.

• Abandoned Mines: Shafts could be dug into the mountain, connecting to partially or fully depleted veins. The entrance might have caved in so long ago the mine is not visible from the surface. The miners may have dug several layers deep. A fierce beast could have scared off the workers leaving picks, ore, and un-mined veins for the taking.
  
• Urist Rogers in the 4th Century!!!!: Perhaps a great dwarven champion from the age of legends mysteriously disappeared, only to be released from its rocky prison by your miners. This hero might offer to serve your fortress as its defender. Eventually, however the wonders of this new world might seduce the champion away. Alternate versions include ax wielding murderous dwarves frozen in ice or Spirits of Fire encased in a private ash or bauxite prison. Mix and Match.
  
• Buried Ruins and Battle sites: Sections of ruins or battlefields from previous eras could be buried in various locations to be unearthed. This could be used to add detail to the otherwise bland non-mountain biomes and to maybe reveal unknown pre-history legends. In these ruins you could find tools, valuable artifacts, secrets to lost recipes or technologies, and remains as well as potential dangers from monsters, curses, or other threats. Imagine the engravings! Adventurers would enjoy hidden trap rooms guarding artifacts or other treasures.
  
• Buried Treasure: Arrrr! Dead dwarves tell no tales. On islands and other coastal areas, adventurers or fortresses might unearth some buried treasure.
  
• The Terror Below: ^ It has been suggested that deep below there may be entire caverns filled with (maybe evil), living, writhing mass of ooze that could be exploited, built into, or otherwise used. Discussion begins here.
  

Magma Features
Ah the joys playing with molten rock. Magma seems to be one of the most hotly (ahem) sought after resources. It has so many uses from free fuel, to black wands of death, population control, and goblin disposal. Here are a few more uses/suggestions:

• Repopulate Magma Pipes from the Bottom Tiles.
  
• Magma "Aquifers": Deep underground entire layers could be composed of magma. This provides another option for metal crafters, even in areas with no vents or pools. Magma creatures could migrate on and off the map much like surface creatures do. These layers would link into existing magma pipes as well. HFS would pierce the mantle much like magma pipes pierce water aquifers.
  
• DEEP_MAGMA Biome: A revisit to the old "Hell" Biome entry. This tag is used to differentiate the normal SUB._LAVA creatures from the real nasties that live deep in the mantle (ie HFS).
  
• Branches off Magma Pipes: Instead of being a single column, magma pipes could have smaller branches that break off, much like the 2D cave river and chasm, or the HFS veins.
  
• Lava Tubes: ^ A tribute to the days of the 2D Magma river, lava can occasionally flow horizontally.
  
• Kimberlite Pipes: ^ Not really a magma structure, but a result thereof. Kimberlite naturally occurs as vertical cone or carrot shaped pipes, not as horizontal veins. Perhaps an algorithm similar to the one that creates magma pipes could generate a kimberlite pipe, with the occasional diamond cluster at its core.
  
• Eruption!: Already in Toady's notes, but mentioned here for completeness. Volcanoes, magma pipes, and any new magma features connected to off-site magma sources could rarely experience eruptions. These events would likely involve earthquakes as well. New layers of volcanic strata could also be formed.
  
• Lava Caves: ^ ^ Multiple types of interesting cave formations can be made by the cooling of lava, movement from one area to another, or the destruction of encased objects as the lava cools.
  

Water Features
One of the classic features of DF is the joy of playing with the water. Now here are a few suggestions to make this even more enjoyable.

• Repopulate Cave Rivers from Upstream/Source Tiles.
  
• Artesian Wells and more realistic Hydrology: A number of features, mechanics, and possibilities were outlined in this post.
  
• Variable Rate Aquifers: * Instead of universally pushing massive amounts of water, aquifers could generate water slowly.
  
• Natural Springs: Small columns of rock much like an aquifer, but that generate only a small amount of water. By tapping multiple springs you could provide a fort with a water supply.
  
• Connect the Cave and Surface Rivers: * In Worldgen Surface Rivers could suddenly go underground instead of carving a path, and Cave Rivers could emerge at the heads of brooks, or as waterfalls. A surface lake could be fed or drained via Cave River. By the same token, underground ponds, lakes and rivers can be interconnected.
  
• Seasonal Floods :  ^ * (I)  Cave Rivers could flood just like the 2D river did.

• The Great Undersea: Drawing from the works of Tolkein, the entirety of the world may sit on top of a great cavernous ocean. Mixing in a little Jules Verne and a lot of Hollywood, the shores of this sea could be populated by a lost civilization, dinosaurs, or pure evil.
  
• Underground Lakes / Open Areas in Aquifers: ^ * Aquifer layers could have pockets already hollowed out and filled with water. These pockets could extend multiple levels, including above and below the aquifer. Chambers above an exposed aquifer could be merged with cave ponds conceptually.
  
• Sand on Cave River Banks: When generating the cave river, some of the banks could have occasional sand floor tiles for glass lovers.
  
• Mud/Sand Flows: All the dangers of a water flow, and none of the benefits. Useful for generating sandstorms, quicksand, flash floods, etc. When the flow "dries up" it leaves soil much like magma leaves obsidian.
  

Subterranean Creatures:*
The player could encounter unique underground civilizations and interact with them , perhaps in the same ways as surface races. They could be found as ambushes, lairs, through tunnels, chasms and caverns. Goblins, batmen, ratmen, and antmen are all good choices. The inclusion of Orcs into DF has been suggested as well. All manner of albino, cave adapted surface species could be encountered.

• Repopulate the Chasms
  
• Bring Back Chasm Raids: For those unfamiliar with the 2D Chasm it used to be far more dangerous, and would generate raids from chasming refuse. The final solution was to "chasm" a great deal of magma..
  
• Truly Subterranean Dwarves: With all the possibilities of underground connections, it could be possible for dwarven civilizations to not be connected with the surface world directly. Their caravans would travel underground, their outposts settled in caverns and chasms, and trade links to surface races non-existant or extremely limited. It could be possible, for example, to begin play at the bottom of a deep chasm connected via tunnel to the mountainhomes, with the intent of mining up to open trade with the surface races.
  
• The Antmen: ^ The current favorite choice for an underground civilization. Antmen would have extensive, seemingly random burrows. The hive would of course be ruled by a queen, protected by soldiers, populated by workers, and contain chambers of eggs and/or larvae (which could be edible). The ants could cultivate a "unique plant" or aphids. This could be traded for, or potentially stolen by a hostile fortress. It has been suggested that antmen, by in large, to a be a more passive race. they would not attack unless provoked.
  
• The Ratmen: ^ These rodent rascals could burrow in dirt, creating warrens similar to, but smaller than Antmen. Unlike Antmen, however, they would be actively hostile, seeking to find ways into your fort. Much like goblins, they could also be prone to stealing children, howver in this case, to eat.
  
• The Gremlins: These little jerks lack their 2D capabilities for mayhem. Giving them the ability  to burrow or having them be dug up by miners would bring them back into their glory. Further mischief they could cause includes eating your food, drinking your booze, breaking mechanisms, or releasing chained/caged animals in addition to their traditional love of levers. A further list of annoying behaviors can be found here.
  
• The Trolls: Adversely affected by the sunlight, the true troll "civilization" dwells underground. These trolls are much stronger, smarter, and better organized than would be expected. With low technology, massive appetites, and barren caverns their population remains small. They are also plagued by goblin kidnappers taking their young to serve as living siege weapons, or alternately, they are actually the outcasts : 1 in 100 goblin children born abysmally mutated.
  
• Lair of the Spider Queen: Far deadlier than the dreaded (or coveted) Giant Cave Spider, an evil race of slave herding intelligent spiders dwell deep in the underground. A detailed write-up can be found here.
  
• The Nest: Even though the legends record the last dragon/hydra died in a battle two hundred years ago, an excavator could discover a nest, its entrance long since buried. Inside there could be hatchlings or even eggs. Do you seal it up immediately, or take a chance to have a pet dragon for your arena?
  
• Burrowing Creatures:The ultimate threat to a Dwarf Fortress has a taste for dwarf flesh, big claws and an irresistible urge to dig. Discussed many times in many threads, this remains one of the most common and most controversial suggestions. Distinction could be made between soil burrowers ("Giant Badger" anyone?) and Stone Burrowers ("The dread Rock Worm"). To prevent excessive tunneling by minor creatures, some species (like Gremlins or Rats) could dig tunnels too small for dwarves to see or enter, but with entry holes that can be built over to seal them. Yes, for the record, this includes Sand Worms.
  
• Endolithic Biome: ^ * ^ Organisms such as slimes, jellies and oozes could live in the rocks, posing a random threat to miners. The deeper you delve, the more dangerous the creatures could become.
  
• Living Rock: (I) A true endolith, perhaps a "breed" of rock exists that repopulates itself using a system similar to Conway's Life algorithm. Providing an endless amount of stone, it could be a rather large nuisance: blocking tunnels, filling rooms, and potentially encasing sleeping dwarves.
  
• Rock Creatures: Ranging from normal size all the way up to a Rock Titan. They might have an outer shell of rock and might even eat rock to get the minerals needed to form the large shell deposits. Obviously they would need to eat flesh as well, dwarves being the preferable light snack. One interesting twist could be that one of the smaller rock creatures uses the same icon as a stone, with the same color as the rock it was mined from. "How many times do I need to designate that chunk of olivine to be dump...you know the one next to all the corpses!"
  
• Fossils/Undead: One great suggestion was the possibility of digging up the fossilized remains of a long dead creature. These fossils might have some use/value as components or materials. The could also be undead with much higher DAMBLOCK than default skeletons. Other possible encounters include excavating a long forgotten tomb, or coming across a hidden city (maybe even an outpost of a civilization) inhabited by vampires or other sentient undead.
  
• Lost Worlds: One excellent way to add flavor to large expanses of caverns would be the inclusion of rare, extinct, or unique biomes. Perhaps dinosaurs roam the place, or an entire cavern is populated by savanna cats complete with tall grasses. Other interesting effects include mind altering magic, suspension of normal "physical laws" or other pocket dimension type stuff.
  

Underground Flora:
Despite the lack of light, it is clear in the DF universe that plants can indeed flourish underground.

• Stone/Ore specific species: It has been suggested that some underground flora should be unique to specific types of stone or metallic ores, perhaps gaining unique properties from their environment. Discussion begins here.
  
• Un-Muddied Plants: More of an aesthetic request, but herbs and tower caps growing in a muddied cavern could be displayed as green to develop contrast and appeal.
  
• Cave Rivers/Ponds with existing Tower Cap Forests : Tower Caps could already be growing around cave ponds. In addition, a cave river could occasionally open into a long cavern with raised banks (perhaps with real dirt) on which forests grow.
  
• Glowing Fungus: When the lighting arc goes in, certain breeds of fungus could give off light. These could be harvested by Dwarves as portable, smoke-free light source. However, it is possible that the fungus might enjoy the taste of a snoozing dwarf.
  
• Subterranean Extracts: In many of the alchemy, magic, and healing threads it is clear that the option to collect rare herbs for extracts (like Kobold Bulbs or Valley Herbs from the surface) found only underground is a great option. Similar to Quarry Bushes in the 2D version, they may not be found in all fortresses.
  
• Cave Kudzu: (I) There could be a nasty fungus or plant with an accelerated growth rate that rapidly fills all tiles it can get to (and block access). You could of course, chop it down with axes. Maybe there could be a "mother" fungus tile that, when chopped down, stops the growth.
  
• Excavating Plants: Ranging from a totally new life cycle for the Tower Cap to a new fungus that fills a cavern and grinds away at the cave walls. Discussion begins here in the thread.
  
• Clinging Plants: Hanging from the walls, or even the ceiling, some cave plants could grow in normally inaccessible locations. These creepers could block access to the stone they cover, requiring removal before getting to the goodies underneath. A fun (and mean) variety might prefer the ceilings of large caverns, and grab passers-by with its long, blood sucking thorn covered vines.
  
• The Madness Plant: borrowed from a very old precursor to this list, this rare plant spreads pollen in close proximity. This pollen causes insanity in sentient races, leading to abnormal behavior, mood swings, and maybe even psychotic rages. Best to try to just wall it up and build around it, or come up with some way to kill it from a distance (cave-in, pour in magma, encase in ice, etc).
  

Meta-game Modifications:
Some aspects of the game may have to be augmented to integrate the above suggestions.

• Room to Build: More of a guideline, rather than a suggestion, there should always be enough room to actually build a fortress (despite how subjective that is). When possible, a feature should telegraph its presence out to a realistic distance so the fort can be well planned. IE Miners might here the sound of rushing water, or here a hollow thump in time to build around the feature.
  
• Visible on Site Finder/Embark: A number of players are concerned about finding or avoiding this new content. Coupled with the axiom of "Room to Build", these new features could be marked on the embark/site finder interfaces. Most could be lumped with caves as "other features", but some of the more dramatic ones (ie large caverns, lava tubes, oil aquifers) would need extra attention.
  
• Controls in WorldGen: When appopriate, there could be controls in the world gen parameters for the frequency of major underground features (ie rivers, caverns, magma or oil aquifers, etc).
  
• Settling on Tunnels: Dwarf Fortresses should be able to embark on sites accessible via underground tunnels connected to other Dwarven Fortresses. The wagon and initial 7 would spawn inside the tunnel.
  

[/list]

[penguinofhonor]

Behold, concept art for a tower cap forest!

[Dame de la Licorne]

Also, for rivers etc. that link off the map, if the creatures there could migrate on/off the map, it would be more interesting and provide more of a challenge.  Not sure how it would work for magma pipe dwellers, since they don't link off the map.  But re-populating of the rivers etc. would be excellent.

I really like that tower-cap forest, btw.  Now if I could only see it in-game...   

[penguinofhonor]

I officially love DFPaint.

And some Magma Creatures could come up from the bottom of the Magma Pipe. This would give some benefit to Magma Pools.

[Puzzlemaker]

I really like Endolithic stuff.  That would be really, really cool.  Urist cancels dig:  Slime

Underground civilizations... Sort of.  I was thinking more like tunnels that animals dig (Like you could run into an Ant-Men nest) and many, many more natural caves that things can live in.

Magma Aquifer... Hell yes.  That would be grand.

[penguinofhonor]

I like the idea of having huge antman nests. Like, huge. At least as big as a fortress. Example:

Urist McMiner cancels dig: interrupted by antman
I decided not to draw the ground texture because that takes too much time. Anyways, the gray ones are the normal antmen, the red ones are grubs, and the purple one is the queen.

[Othob Rithol]

Nice conceptual art PoH.. could you maybe do one like the cave pod, but with a river? maybe a waterfall?

I'll tweak the description of underground civs to suggest lairs.

[Puzzlemaker]

Expand it even further then that.

For every ___Men, have additional stuff in the Raw files.

Like...

Location of their Home.  This should maybe be tied into their Biome, or something.  The difference is, these define where in a local area they decide to live, not what the Biome their home is in.

Cavern:  They live in an open area, but it's enclosed
Chasm:  They like to live in Chasms
Magma:  They enjoy living near magma
River:  They like living near rivers
Lake:  They like living near lakes
Water:  They like living near water
Underground:  They enjoy living underground
Lava:  They enjoy living near lava

Of course you could combine multiple of these.  And there should be a REQUIRED tag for some of these.  Like for frogmen, water would be required.  They would also have a lake tag, meaning they like lakes better then any other type of water, and maybe the underground tag too so they would like living underground more, but the only requirement is they have to live near water.


Also, how they change where they live:

Burrow:  They dig tunnels and stuff, and live in them.
Nest:  They live in small nests, where they hoard stuff.  As in, tiny piles of junk that they defend.
Decorate:  They decorate the area they live in with various things, like bones.  Maybe additional options for this one, like Webbing, Spikes, Slime, and other decoration.
Room:  This one is a tag to define different rooms they would have, if any.  Like Hoard, Leader_Room, Nursery, etc.

And the way they are organized:

Loose:  Loose organization.  Pretty much means they tend to be in the same place, but they don't work together really.
Territorial:  They attack each other and anyone else who bothers them.
Groups:  They tend to separate into separate groups
Hive:  They all work closely together, tending to attack and retaliate as one.

These can be combined, to an extent.  Loose and Groups means they form loose groups scattered around.  Territorial groups means they form groups that don't like each other.  Loose Hive would mean they are more scattered around, but they still act as a group, Territorial Hive would mean a Hive that really doesn't like anything else around, etc.


So, for the Antmen example...

[liVES:UNDERGROUND:REQUIRED]
[liVES:CHASM]
[liVES:CAVERN]
[EFFECTS:BURROW]
[ROOM:LEADER_CHAMBER]
[BEHAVIOR:HIVE]
[BEHAVIOR:TERRITORIAL]

I think that would add a lot, if all the ___Man animals got these tags or something.

Now, the difference between those animals and Kobolds is they wouldn't have any effects on worldgen or anything, they would be considered animals, like a pride of lions.

[Block]

I would like endless digging. You know, when you dig deeper the game just generates more and more of the map. Especially if much of the suggestions here are added you will need more space underground.

[Puzzlemaker]

I would like endless digging. You know, when you dig deeper the game just generates more and more of the map. Especially if much of the suggestions here are added you will need more space underground.

http://www.bay12games.com/forum/index.php?topic=4833.0

[penguinofhonor]

Here's some cave river.

[Othob Rithol]

Now that pick takes me back...I miss my nile farms, the limestone fortifications, and the pitter-patter of little cave crocodile feet in my forest...

Adding Dynamic Fortress Size to list

[isitanos]

Slightly edited copy/paste of one of my old posts.

Compared to the 2D version, the "dig deeper" feeling is completely lost: you can dig stairs straight down to the bottom of the playable area in no time, and 1- there's nothing there most of the time except an artificial limit, and 2- it's not deep at all.
If there's a lower limit, getting there should be exciting, dangerous and possibly rewarding: there should be an incentive to dig deeper to get better minerals, ancient artifacts, etc.

Personally I think that you should be able to dig down indefinitely - but pockets of harder rock would slow you down, and get more plentiful as you go deeper; and there should always be something to find down there. Between underground rivers, cave systems, ancient mines, underground (friendly or hostile) civilisations, magma, demons, spirits of the deep, Cthulu and his friends, or whatever else, there should be something in the deep at almost every map location. This is a fantasy world, after all, and most of all it's a game, i.e. supposed to be fun. Now, we might want to keep a few safe-ish locations so that people who actually like to play sandbox can get a safe place to play.

Some other ideas gathered from that old thread: Underground tombs, Cave systems full of undead lizards, Aquifer with water demons...

EDIT: Oh yeah, and definitely digging creatures. These would be a big potential threat if they dig fast, and goblins or other hostiles could purposedly bring some along with them when they siege/ambush. If you encounter them underground, it could lead to interesting territorial warfare.
Heh, and it would be interesting if they laid traps in their own tunnels...

[Techhead]

A small cave system full of antmen is a great idea.
1. They prefer to build caves old-style.
2. They only dig through soil.
3. They have less obvious holes in the ground, and will not hesitate to occasionally build a second or third.
4. Antmen-queens are like GCS without the webs.

[Duke 2.0]

 Also, note: Sometimes in underground caves, there will be crystals. Just random clumps of gems like we all enjoy seeing in fantasy novels.

 Kinda like the following link:
 http://www.lifeinthefastlane.ca/wp-content/uploads/2007/12/crystal_cave_giants_2.jpg

 And when lighting is added, they could glow!

 Then you would have boulders that have moss growing on them, minerals that grant strange things when dug(Mining for fish, anybody?), horizontal magma flows, mineral patches with more varied shapes than veins, large areas and small clusters.

 Although this is more of a site thing, I would like for there to be more varied sites. This does somehow tie in with underground diversity. Mines, fortresses, dungeons, farms, small towns, large cities, walls, all kinds of things that can be buried underneath some sedimentary layers. That would be kinda cool, having a wall built by humans in an old war uncovered during the construction of a dwarven fortress.

 Geysers, springs, magma pots, anything that can be a liquid underground can exit the surface by many means.

 Every feature in the following link:
 http://physics.uwstout.edu/geo/geology_prof2.jpg

 Cave systems that are more... well interesting. Let there be more vast caverns, small rivers running through the system, with many levels flooded. We want somewhere for Olm's to live other than long water-filled tubes!

 Finally, harder minerals. Currently a legendary miner can dig through any stone later with ease. Heck, even halfway between novice and legendary dwarven can sail through soil with ease. We need stones that take ingame weeks to dig through. Or even minerals that can only be breached by specific picks. Copper picks won't cut it anymore when it comes to stones. Then you would need to invest in either picks made with better metals, or specific equipment for dealing with specific stones. You could make a fortress in a patch of soft stone in the middle of a large field of hard stone, which would be like our current walls. Only the specific way they are designed to be deconstructed will work effectivly.