Olympia: The Age of Gods PBEM (May 4 1999)
The deeper a mine becomes, the more frequently cave-ins or other accidents will occur. The chance of a cave-in occurring in an unimproved mine shaft is (depth/300)% per month. That is, a depth 300 mine shaft has a 1% chance of collapsing each month. A depth 1800 mine shaft has a 6% chance of collapsing each month.
When a mine shaft collapses, it destroys itself and all lower mine shafts. Any nobles in the collapsing shafts will evacuate, but will lose 1-20% of all belongings (including men and beasts) for each level of mine shaft they must evacuate. A noble in the depth 1500 mine shaft of a mine when a cave-in occurred on the depth 800 level would need to evacuate through 7 levels of collapsing mine shaft, losing 7-100% of his goods. Nobles themselves take 1-20 points of damage for each level they evacuate, so that in this case the noble would also suffer 7-140 points of damage.
The chance of a mine shaft collapsing can be reduced by adding wooden or iron shoring to the shaft. Wooden shoring reduces the chance of a mine shaft collapse by 2.3%. Iron shoring reduces the chance of a mine shaft collapse by 4.6%. Regardless of shoring, there is always a 0.2% chance per month that a shaft will collapse.
In addition to the danger of cave-ins, it has been reported that at great depths miners occasionally encounter various terrible monsters, most especially [310] balrogs.