Spelunking

A character with this proficiency has a thorough understanding of caves and underground passages, including their geology, formation, and hazards. The character generally knows what natural hazards are possible and what general equipment a spelunking party should outfit itself with. A successful proficiency check can reveal the following information:

• Determine, by studying cracks in the walls and pebbles on the floor, sniffing the air, etc., the likelihood of a cave-in, flash flood, or other natural hazard. This only works with respect to natural formations, and is negated if the natural formations have been shored up, bricked in, or otherwise tampered with.
• Estimate the time required to excavate a passage blocked with rubble.
• While exploring extensive underground caverns, a successful check reduces the chance of getting hopelessly lost when confronted by multiple unmarked passages, sinkholes, etc. to a maximum of 30%, assuming good lighting (see DMG Table 81-82).

[Complete Ranger's Handbook]