The underground chambers are actually way up in the sky. If you manage to clip out of one it's a long fall.

I dont think it will be possible to add tunnels as a feature, the way the game generates and maintains the land and water areas makes for quicker drawing but it does not lend itself to defining anything under the land... it basically just records the surface of the land, and the level of the water.
Underground items are seperate objects in the database.

You can build small caves if you dig under a large rock (the rock object, not rocky ground), but thats about it, that and basements under your house, with the floor as the roof (note floor tiles do not count as a roof when it calculates sheltered status).

Has anyone ever tried enlarging the underground burial chambers? Probably doesn't work. This post just made me think.

You can always use those as is after cleared. Skels only appear to respawn again outside near entrance.

there's some cool workarounds to this i've found. 1st is under the 2nd Boss Area since its a
large stone plate, 2nd is in the mountain biome you can find rocks often in clumps of two or three and those make for good roofs. those are the ones i've experimented with.

Originally posted by redgreen999:

Has anyone ever tried enlarging the underground burial chambers? Probably doesn't work. This post just made me think.

You can always use those as is after cleared. Skels only appear to respawn again outside near entrance.

you sadly cant dig the ground in there, as well as build inside them

You could make a trench and then cover it with wood / rock piles or maybe a hide of some sort. Certain biomes can offer some decent rock formations to excavate under.