On April 26, 1986, in Pripyat, Ukraine, Chernobyl Reactor #4 suffered a power increase, which caused the whole plant to burn. On the night of the incident, Chernobyl's staff ran a safety drill. An automatic shutdown was supposed to happen in case of low water levels. But operators, who lacked proper training, blocked the automatic shutdown mechanism, because they thought the shutdown would abort the test. The coolant started boiling in the reactor, and reactor power slowly increased, which caused Reactor #4 to explode.