Also known as black coal, bituminous coal is an organic sedimentary rock formed by metamorphic compression of peat bog material. In the United States, it is between 100 and 300 million years old and formed during the Pennsylvania sub-period of the Carboniferous Period, when a vast network of swamps covered large parts of North America, including the Appalachia region. Much of the organic material created in these wetlands accumulated into thick layers of peat that were buried faster than they could decay. Bituminous coal is relatively soft and contains a tar-like substance called bitumen or asphalt. In the coal mining industry, it is known for releasing fire damp, a dangerous mixture of gases that can cause underground explosions, so extraction requires the highest safety procedures, including monitoring of gases, good ventilation, and vigilant site management. Such dangers led to disasters including the Mather Mine explosion of methane and coal dust on May 19, 1928, which killed 194 miners, and the Robena Mine explosion of methane and coal dust on December 6, 1962, which killed 37 miners. Bituminous coal is 60 to 80 percent carbon, with the remaining content made up of macerals of vitrinite and liptinite, sulphur, hydrogen, and water. Depending on its use, it is graded by plasticity, volatility, moisture content, and the presence of other minerals including ash content. Varieties include smithing coal, a high-quality forge coal free from ash, sulfur, and other impurities; cannel coal, also known as oil shale, a terrestrial coal with low mineral content used as grate fuel in home fireplaces and sometimes by artists for jewelry and ornaments; and coking coal, also known as metallurgic coal, used in steel manufacture because of its low volatility and lack of ash.