Cave ins, Fissures that open under your feet, earthquakes, creatures trapped since ancient times unleashed by said earthquakes, Natural inhabitants of the surroundings would include spiders snakes centipedes and insects of all kinds, If they delve low enough you could have them reach ancient dwarven strongholds with stone golem guardians and such. Steam geyesers that are hot enough to flay the skin from a mans hide, chambers that flood suddenly when an underground stream breaks through to them.
Endless possibilities.
Endless possibilities.