What can you build in a huge underground minecraft room in survival?
A large warehouse, a bunker, a farm, or even better an underground house
You could build underground rails to a nearest village if it is not far away and thus build a subway station.
I would build a gate to hell, it depends on whether that fits your topic. Anyway, was the only thing I dug a huge hole for. There are handsome Oni-style Nether portals on Google. You can quickly find inspiration there
I would interpret everything in such a way that you never have to go back to the surface.
Hang lanterns from the ceiling and create monstrous pumpkin farms.
I also like to breed critters in caves because they can barely run away. Outside they bend over fences every now and then.
Recreate Dwemer Dwarf Ruins from Skyrim.
The power comes from the cellar! ^^
