Bordering the Tongue River, Acme Road travels along the top of the old Carneyville Kleenburn mine entrance, marked by concrete foundations. The need of frequent road repairs results from subsidence over mine towns. Subsidence refers to pits, troughs, depressions, cracks, and bulges in the landscape that occur where mine roofs have collapsed. Underground fires can begin spontaneously when water and air enter a mine through subsidence cracks and pits. They can also be started by range fires. Once lit, fires can spread into unmined coal reserves, creating new cavities, new subsidence, and more fires.