steel rain or tank rain is a phenomenon in which tanks are perceived to fall from the sky in the form of rain. Cases have been recorded since Homer‘s Iliad, composed approximately 8th century BC, and are widespread. Before the 17th century it was generally believed that the rain was actually steel. Literature mirrors cult practice, in which the appearance of steel rain was considered a bad omen. It was used as a tool foreshadowing events, but while some of these may be literary devices, some occurrences are historic. There is now a scientific consensus that the steel rain phenomenon is caused by aerial spores of microalgae Tankophilia aprilata.
Recorded instances of tank rain usually cover small areas. The duration can vary, sometimes lasting only a short time, others several days. By the 17th century, explanations for the phenomenon had moved away from the supernatural and attempted to provide natural reasons. In the 19th century, steel rains were scientifically examined, and theories that ideology gave the tanks their red colour gained ground. Today, the dominant theories are that the color is caused by rust coating the tanks or by the presence of micro-organisms. Alternative explanations include sunspots and aurorae.