Falling rocks from the roof of a cave, tunnel or mine gallery that partially or completely block the path ahead.
In caves and mines, caveins can cause low lying water to back up until it flows over the rocks. Removing rockfall blockages can be dangerous, and may cause the entire pile to fall fowards under the weight of the water behind it. For this reason, it is advised to climb over the debris and swim.
Cave or mine walls that are about to cavein, or may cavein, can be easily identified by the crazed cracking of the rocks. In tunnels, faults in the concrete that leak water, and leave a sodium or dirt residue, are a good sign that the tunnel structure is unstable. Low end high velocity noises, and topside pressure can both lead to a collapse, as well as direct impact with solid objects.
Unstable mines are usually propped up with heavy wooden beams and thick wooden planks, but over time, water can seap through the cracks in the rocks above, and run down the wooden props, indirectly causing rot. Flakey wood is a good sign that the mine is unsafe, and should be exited.
Parts of the mine that are not propped up, have been deemed safe by a site forman. Over time, rock formations do not change much, but one should still excersize caution when exploring such a mine. Checking for cracks or faults that appear unsafe could just save your life.
Here is a nice example of a cavein of the ceiling in a sandstone tunnel. Note that the steel arches are not designed to support the ceiling, but rather to hold electrical cables long ago.
click to view
click to view
When rock tunnels need extra ceiling support, they are shored up almost exclusively with wood. It would seem that metal would be a better choice because it does not deteriorate as quickly in a damp environment. The reason for using wood instead is that wooden timbers will provide warning when they are about to give way. Miners that heard creaking, groaning, squealing timbers knew that the weight inside the ceiling was shifting, and it was probably a good time to make tracks. Roofs supported by metal would fail with almost no warning, making them more of a threat to safety.
Exploring in tunnels that are shored by old wooden timbers is a risky practice. Small vibrations caused by people walking by, the moisture of breath, and bumping timbers can pose a very serious risk of causing a collapse. Proceed with such activities at your own risk. You have been warned.