Sinkholes!

12 Fossil marine creatures or monstersd12 Common What’s in the Bottom
1 Flooded (if near sea, possibly connected)

2 Pile of rubble in a cave

3 Vast interconnected cavern system

4 Cave full of prehistoric animal remains

5 Carved tunnels like a mine or a dwarf transit tunnel

6 Giant burrow complex

7 Mouth of a titanic monster

8 Cave art gallery with ritual remains

9 Ancient underground tombs

10 Cave with growing vegetation  

11 Cavern with a giant fungus

12 Underground lake or river

d12 

Rare Whats in the Bottom
1 Village, possibly subterranean peoples
2 Ruined ancient temple or shrine
3 Dungeon complex
4 Giant crystals
5 Nests of rats or other vermin 
6 Wreckage of an ancient vehichle
7 Malignant spirit burial ground
8 Ancient buried town
9 Vast magical machine complex
10 Lava pool or burning gas vent
11 Bubbling toxic pool emitting vapours
12 Prehuman monolithd12 Common Creatures
1 Swarm or giant bats
2 Swarm or giant bugs
3 Swarm or giant spiders
4 Swarm or giant centipedes
5 Swarm or giant frogs
6 Swarm or giant scorpions
7 Swarm or giant rats
8 Troglodytes or reptilians
9 Cave folk or apes
10 Gnomes or goblins  

11 Dwarves or trolls
12 Elves or orcsd12 Common Finds
1 Dead explorer
2 Skeletons in a pile
3 Body with broken legs
4 Semiprescious stones 
5 Metal ore seam 
6 Mining equipment 
7 Crude map or writing scratched on the wall 
8 Remains of ladders or stairs have fallen in
9 Pool of water 
10 Coal seam or peat deposit 
11 Garbage mound
12 Bat guano mounds and polluted pools

d12 Rare Creatures
1 Cave clams
2 Cave fisher or giant lobster

3 Ogres or a giant

4 Ancient mummies

5 Subterranean mutants

6 Giant albino cave apes or bears

7 Giant cave lizard or turtle

8 Deranged adventurers turned cannibals

9 Albino dinosaurs with reptilian masters

10 Frog or toad folk with giant pets or insect herds

11 Ancient golem or juggernaut

12 Vampire or ghost, so old is bestial horror

d12 Rare Finds

1 Sacrificial victim remains (animal and/or humanoid)
2 Eggs, one giant or hundreds of large ones
3 Grotto with crystalline growths and mineralised skeletons
4 Crystal pillars from prehuman civilisation
5 Magical pool with strange potion-like powers
6 Faerie fungi forest with magic schrooms and ecology of critters
7 Sleeping kaiju or primordial giants awaiting the apocalypse
8 Remains of a dead primordial being with metal bones or blood
9 Dragons lair entrance
10 Pack lizards with baggage looking for owners
11 Vein of precious stones or metal
12 Connection to ancient subterranean trade rout to underland