QA-POS-1: How do I mark a disc in an inaccessible location below the playing surface like a crevice? Is there a penalty?

The rules that apply to a disc above the playing surface also apply to a disc below the playing surface. If you can locate your disc in the crevice and no reasonable stance can be taken there, you can mark your lie directly above it on the playing surface without penalty. If the point directly above the disc is in the air or within a solid object, mark your lie at the first available spot back along the line of play.