Okay, that cavern I found eastbound, which I have formally named it the "Sandy Sinkhole Mines", is a hell of alot deeper than it initially appeared. Here's a hint for you spelunkers. Gravel is quite valuable. They seem to link to other cave systems and have a habit of hiding ores.
In SSM, I found a few pockets of iron, much gravel, and hidden behind a gravel pocket, a block of gold ore. Unfortunately, I have no iron picks right now. It'll have to wait. Nearby the gold ore, there's a small cave waterfall (already breached when I found it), and thanks to caving in gravel pockets, I found more cavern links. This place is huge. I have upped the difficulty to medium when I started spelunking. You know, to make it fun. No monsters yet, but I fear a dungeon could be near.
EDIT:
Crap, I hear hissing.
EDIT EDIT:
YAY! More gold!!!
EDIT EDIT EDIT:
Okay, that cavern is larger than Spelunker Mines. No dungeon, and I don't know the source of the hissing. However, I now have a metric crap-ton of coal. I've reset the difficulty to peaceful after A FREAKING CREEPER WELCOMED ME!!! I killed it before it had a chance to detonate, but still. Holy crap.
I have too much crap on me, I lack an iron pick to mine the gold and anything else more difficult to mine, and I want that gold while I remember the map of the cave.
EDIT EDIT EDIT EDIT:
I got 18 gold blocks, and found more trace deposits of iron, and yet again, tons more coal and gravel, and risked a few dirt pits. I also got lost in that cave for a few moments. Holy crap. I may have to do an update for the underground map.