Abandoned mines are closed for a number of reasons, including protection of unwary visitors and protection of wildlife habitat. Even when mines appear to be stable, there can be many dangers that are not obvious. These include deep water, shafts covered by rotting planking, unstable shoring, and loose rock. The most common hidden danger is low oxygen levels. The oxidation and decomposition of sulfide minerals robs the air of oxygen, while the decomposition of organic material can produce carbon dioxide. In either case the oxygen level can be reduced to a level that can cause an unwary visitor to fall into unconsciousness an die. Carbon dioxide is heavier than air and will sometimes form an invisible layer a few feet thick near the floor. When visitors walk through such an area the carbon dioxide can be mixed into higher oxygen air above reducing the total oxygen level by dilution. When working in mines we monitor air quality constantly and if there is any question, artificial ventilation is used in the work areas.
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