Some ideas for natural phenomena in an old mine:
- old bridges likely to break => need some ideas to cross a chasm. Players get some fine ideas unless they simply use Dim.Door, Fly, et al. Depends on your PC levels.
- release of toxic, possibly inflammable gas. Roll saves secretly. Maybe give some hints: strange smell, hissing noise. PCs need to find a way to deal with the situation.
- party is separated by a collapsing tunnel, maybe one PC is half-buried and needs to be saved while monsters attack. BUT: splitting the party may be quite a headache for the DM!
- tunnels are flooded with ice cold water (clear, or full of particles) => maybe poor sight, danger from cold water, length of tunnels unkonwn (but maybe there are some bubbles with air)
- large groups of foes need to passed without getting noticed, or guard posts must be avoided because foes there are obviously tough!
- old equipment can be reactivated to solve a problem (like old lorries or so).
Watch some movies for ideas:
- Indiana Jones movies (nice traps and obstacles, especially in Temple of Doom, and the Last Crusade)
- The 13th warrior (party makes the way through the caves)
- Sanctum (divers locked in caves due to flooding)
- The Cave (horror movie)
- Lord of the Rings (Mines of Moria)
...