I decided to go on a bit of a supply run importing wood from Starter Mines, but after farming some trees (and establishing a reed farm as well), I did a bit of exploring of the Great Plains, and found a small cave with a pocket cave near it (now made as a camp site and a shipping relay point from the more important thing I found).
A little more exploring later, I found a huge-ass cavern, now titled Spelunker Mines, with the pocket cave as Miner's Hideout. Spelunker's is very well hidden, and I didn't notice it until I explored the Plains. Found some ore and tons of coal, and more than enough gravel. Considering the difficulty of normally finding it, I made a torch, and simple directions to it, with a simple torch path if wandering the plains.
Anyway, the Spelunker Mines are enormous. At least a couple forks were found, and I have barely explored it entirely. I may need some resource chests if I decide to spend a day or 2 in there. Bonus if I can find any magma. I could use another export. I think the UG maps will show a giant glowing spot now, since I found so much. I could maybe check out the UG map and see if I can spot it.
EDIT:
Okay, if I interpreted my cavern map correctly, HO-LY CRAP!!! This cave will take me some time to fully explore, and it should guarantee magma in a few spots. And this doesn't seem to be the only major cave system in the Great Plains.
This'll be fun.
EDIT EDIT:
This is in my SP map. Same one I've been posting about.
CAVE NOTES:
Heading out to the first right led me to another entrance to the cave system. Did a little exploring and gravel/coal mining, and collected a decent amount of material. Now labeled as a back door.
To the left, I have cleared out 2 more gravel pits, and the road also split into 3. The upper path had some iron deposits, as did the fork and the bottom path. To the right is another form in the road. That is to be explored. Fortunately for base camp, I should keep at it for another while, but it's late right now. I'll continue mining tomorrow.