Hey, I just found out that if you like put a lava block somewhere and die in it, and say /back you will come to a cave under it or something that is under.

I'll do an tutorial so you understand it:

First make a square so that you can fill the middle (1 block) with lava.
Then make sure you don't have anything you don't want to lose.
Then hop in the hot tub ;)
Next step : You HAVE to have /back command. Say /back

POOF you are in the lowest cave / hole under the lava.