Light emission by fungi was already known in the ancient world. Although this phenomenon was mentioned only very briefly by Aristotle and Pliny the Elder, naturalists mostly neglected the subject until the early observations were confirmed in underground locations by miners in the 18th century. In 1796 the German naturalist Alexander von Humboldt was one of the first to describe the luminescence of rhizomorphs (thick "root-like" mycelium strands) in the mines of Freiberg (Germany). A bright luminescence of wooden panels and beams in a coal mine was reported, so bright that pit lamps were not necessary. High levels of air humidity seemed an important requirement for light emission and increased temperatures stimulated luminescence as well. Light was emitted by the "plants" (termed Rhizomorpha subterranea, R. aïdela, R. stellata, R. noctiluca) mainly from the tips of the mycelium strands. Today, it is rather difficult to see this spectacle in nature, e.g. in forests, mainly because of increasing "light pollution" of the environment. This report records the perception of this phenomenon in mining locations and discusses the original (mostly German) literature of the 18th and 19th centuries.