You can't railfan the Powder River Basin, in the northeast part of Wyoming, without getting the trademark red mud that is seen covering my truck on your vehicle. The red mud is from the red dirt roads that crisscross all over the Powder River Basin which seem very unique to this region.
The red rock is formed when natural sub-surface fires in coal beds start where erosion has exposed the coal to air at the surface. Because it often contains sulfur, coal may spontaneously combust under certain conditions. Brush fires or lightning strikes also may ignite it. Presently there are at five such fires on the Southern Ute Reservation in Colorado, one of which has been burning since at least the 1930s. In 1999 a Park Service firefighter working in that area was overcome by hydrogen sulfide gas coming from a vent over the subterranean fire. The tribe spent a million dollars in 2000-2001 to have "grout" poured into vents in an effort to extinguish the fire, but it had no effect.
Wherever there is coal it is likely that there will be evidence of ancient subsurface fires. When Lewis and Clark passed through Montana-Wyoming in the early 1800s, they encountered a river bounded by boulders of clinker. They named it the Redstone River. Later explorers, smelling the smoke from coal fires in the area, were reminded of burning gun powder, and gave the stream its modern name: Powder River. Geologists estimate that between 30 and 50 billion tons of coal have burned in the Powder River Basin to produce the quantity of clinker found there, some of which occurs in beds nearly 200 feet thick.