There are three different features that sort of match. The first one, with the best match, is the one Insert_Gnome_Here refers to. This one is displayed with the advanced world gen parameter for displaying caves. I think caves can be disconnected from the caverns, but all the cases I've seen have been connected to the first cavern. A tunnel can very occasionally contain critters (like a kobold nest), and it ought to be possible to find (semi) megabeasts in them, although I've never done so.
Note that a cave like the one above doesn't release spores automatically: they're released only when you discover the cavern (I suspect one way of releasing the spores is to go down the tunnel).
The second one is a tunnel connection between two of the caverns (but not the surface). When it is discovered, there's a message saying you've found a downwards tunnel (or something like that), even if you've actually found the bottom end first.
The third one is similar to the second one (with the same message), but instead of a tunnel it's a vertical shaft that looks like an empty magma tube (and doesn't reach the magma sea).
I'm not away of any way to detect the latter two beforehand.
Barricading a cave will block anything that may be in it, as well as cavern critters (such as FBs) from going out onto the surface. Pouring liquid down a cave sounds rather pointless, as it will pour out in the cavern below and evaporate (the exception being if you're actually trying to drown a kobold nest).
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Topic: Finding tunnels (Read 564 times)