3 CAVE IN. This passage abruptly terminates in rubble. The passage once continued to additional storage rooms, but the way is now buried. The passage may be excavated by a determined party, of course, but the noise of such work will usually double chances of encountering wandering monsters. In addition, there is a real chance of further structural instability and crashing stoneworks. For every ten feet excavated, there is a 50% chance of falling stone doing 2-12 points of damage to workers unless a save versus paralyzation is made to get out of the way at the last instant. After about 120 feet or so, the passage leads to Area 8 of the Grounds.
Rules for mining can be found on page 106 of the Dungeon Master's Guide. The rock should be considered 'very soft' since it is really just the removal of rubble, and subsequent bracing of the tunnel.