U.S. DEPARTMENT OF THE INTERIORBUREAU OF LAND MANAGEMENT
Common hazards include:
Unstable Rock & Decayed Support Structures: The wooden debris beneath your feet may be the only thing between you and a lower level several hundred feet below, or it may be all that was once supporting the rock above your head. These frameworks may have been decaying for more than a hundred years.
Deadly Gases & Lack of Oxygen: Chances are you won't sense or smell a thing until it's too late. Abandoned mines are not ventilated and pockets of methane, carbon dioxide and other deadly gases can form or simply displace oxygen, with no visible sign. When you breathe these gases, your muscles stop responding normally, then your thinking becomes clouded, followed by unconsciousness and death. The same fate may await the family member or friend who attempts to rescue you.
Explosives & Toxic Chemicals: These ingredients of active mining were often left behind when the operation was abandoned. Explosives such as dynamite and blasting caps become very unstable over time and can explode if disturbed. Storage containers, boxes, barrels, and drums deteriorate, allowing toxic chemicals to leak or to combine into highly dangerous mixtures.
High Walls & Open Pits: Not all mines are underground. Often, large areas of the surface have been disturbed to access minerals near the surface, leaving large open pits and/or vertical cliffs (high walls). Open pits can be filled with water which, in turn, can be highly acidic or laden with harmful chemicals. High walls can be unstable and prone to collapse. When approached from the top, the vertical edge of a high wall may not be seen in time or may crumble, leading to a fatal fall.
Becoming Lost & Disoriented: Mines can be honeycombed with literally miles of horizontal and vertical passageways that randomly follow the original ore veins. Complete and total darkness occurs within a short distance of the entrance. Artificial light sources such as flashlights and lamps can unexpectedly break or fail, leaving you deep in an underground maze so dark that you cannot see your hand in front of your face. Although you can cautiously reverse your course in absolute darkness, you may find many side openings that you had not kept track of on the way in - a wrong turn can be fatal.