Why do tunnels collapse?

A tunnel is an underground or underwater (or both) route for the transit of people and/or goods. The tunnel is surrounded by rock or other earthen materials, water or other things, and these all apply pressure on the structure of the tunnel. Sometimes these structures fail.

The failure of a tunnel can come as the result of poor engineering or construction. A "bad" design or the failure of the construction crews to follow a design can leave a tunnel vulnerable to failure. Additionally, there may be earthquakes or other natural disasters, fires or explosions in the tunnel, or other events that the tunnel was not made to withstand, and collapse is possible under the circumstances.