I was exploring the flatland area on my Millenaire world where I made the Item/Sell/Energy-O-Mat tutorial, when I found a weird occurrence of cobblestone.
I went through the hole and found an UNDERWATER DUNGEON! (kinda)
I took a few luminators and tapped the water with them.
Digging out the sand which fell on the chest...
Exposing a single chest with cocoa beans, bread, 2 iron ingots, 2 saddles and redstone dust.