The ghost of "Big Bad John" (find the song) is in the mine. The PCs know him by reputation; he is a hero in local folklore. He can lead the PCs to a "dead end" with a treasure hidden beyond the cave-in.
Gives you a chance to play some background music.
If the PCs tell him about his reputation among the living and give him a decent funeral service - something brief, like reciting "ashes to ashes, dust to dust" - the ghost will pass on to his just eternal reward.
edit: for the funeral they also need the body, which they automatically find if they dig out the cave-in
Other background music: Devo "Working in a Coal Mine". There's an older version with a singer whose deep voice sounds like he really did work in a coal mine all his life.