Yes, I started on a 16x16 (18x18 with walls) mining central... Each floor has 256 blocks which need to be removed, so I slowly dig down into the cave systems. There's something exciting about working on a big project before really going down, instead of just making one ladder going all the way down in 10 minutes.
I ventured into the cave branches I could find, and it didn't seem like it was an epic cave, however. I found a zombie dungeon, and the lowest level was about 92 (which would be like level 41 in a normal world, but in size it makes the cave system extend down to what would be 28).
@Matty: It's a mod, I posted a link to it on the last page, hidden in a "...!!!".