It's not so great mapping out caves, only surface stuff. You can use it to view minerals but like I said the maps are so big it's basically useless for that.
I had a similar moment of awe finding a huge pit in the middle of an otherwise regular cave.
I always carry at least one full stack of wood whenever I do anything now. Having to dig yourself to the surface from half way under the map with no tools because you got lost in a cave is very very frustrating. I would have gladly thrown myself in lava if I could have found some easily.
On another forum someone randomly spawned this world and shared the save file if you're interested (I have bad luck with spawning on flat land and beaches):
[Blocked Image: http://i.imgur.com/iWDPT.png]