Another view of cave-in of underground Union Pacific coal mine, Rock Springs, 1930s

Early coal mines  used the room-and pillar excavation method. Pairs of men created long narrow rooms at right angles to the mine tunnels as they removed the coal. The rooms were about 24 feet in width and hundreds of feet long. The walls separating the rooms were the pillars that supported the roof of the mine and the pillars were supplemented with timber posts to prevent cave-ins.