Underwater room is probably a better description but it’s still pretty cool and you can see squids swimming around in the water above and around you.
Here’s a quick tour and demonstration of a cobblestone generator I built in minecraft. It basically just uses a sticky piston to mix lava and water which creates cobblestone. This is then pushed up through the top of the structure by four more pistons where it can be mined rapidly with a diamond pick. Sure I could just go and dig a hole in a mountain somewhere to get cobblestone but where’s the fun in that?