So I've been trying to find a site that's relatively flat with iron, flux stone, and a river. Resulting in a whole pile of rejected sites and half started settlements. I finally get to a site that looks good and start digging straight down. Usually with only seven dwarfs the game runs pretty smoothly right? Well I was getting the lag and frame rates I normally associate with a 200 dwarf fortress with attached dwarf shower haus. It was bad, particularly when the river thawed. So I was digging down hoping to reach a lake cavern so I wouldn't have to dig a cistern and actually see what ores were available. It was looking pretty bad anyway (just copper ores) then I realized I hadn't hit a cave yet. I found the magma sea without hitting a single cave. In the magma sea was a massive magma fall, like huge (I think it was 30+ blocks across), pouring into some sort of hole that probably went to hell. This explained the lag. Needless to say I won't be building there.
