[1] In room and pillar mining, tunnels are advanced in a rectangular pattern resembling city streets (tunnels), leaving behind blocks (pillars) of coal.
To a miner, a partially completed tunnel resembles a room dug into the coal seam.
An explosive failure is called a “bump.”[2] In the eastern United States' coalfields, bumps are more likely when the overburden is at least 500 feet (150 m); where a strong, overlying stratum, such as sandstone, occurs near the coalbed; and with a strong, inflexible floor.
In the United States, the number of deaths from bumps had dropped off dramatically since the early 1990s, but fatalities are more common in the West where mines often run deeper.
Debate over the cause of the August 6, 2007, Crandall Canyon Mine disaster, which took place 1,800 feet beneath the surface, raised public awareness about coal mine bumps.