The coal mines are closed due to hazards posed by the age of the mines, the gases they produce (carbon dioxide and methane), and their rock quality (the shale and coal in the mines is unstable). In 1980 four young boys from the local area were killed by the methane gas from one of the open mines However, a number of mine openings have been turned into public-access openings which allow visitors to look into the mines and, for some, to walk a short distance underground. The deepest public-access opening is known as Prospect Tunnel. Visitors can enter of this excavation made by miners searching for coal.