The April 1986 accident—the world’s worst nuclear power plant disaster—was caused by explosions at the Soviet power plant, sending radioactive clouds across much of northern Europe. According to the World Nuclear Association, the accident was the result of “a flawed reactor design that was operated with inadequately trained personnel and without proper regard for safety.”

The trouble began at 1:24 A.M. on Saturday, April 26, when Unit 4 of the Chernobyl Nuclear Power Plant, about 70 miles outside of the Ukrainian capital of Kiev, was rocked by two enormous explosions. The roof was blown off the plant and radioactive gasses and materials were sent more than a half-mile into the atmosphere. Though two workers were killed instantly, there was no official announcement about the hazardous blast. It was the Swedes who detected a dramatic increase in wind-borne radiation, and on April 28—two full days after the accident—news of the event was briefly reported by the Soviet news agency Tass.

Two weeks later, on May 14, First Secretary Mikhail Gorbachev (1931-) went on national television and explained what officials knew about the accident. More details were revealed over the following months. The explosions were caused by an unauthorized test carried out by plant operators, who were trying to determine what would happen in the event of a power outage. There were six critical errors made by workers during the testing, which combined to spell disaster. Perhaps the most significant of these mistakes was turning off the emergency coolant system: Once the test was under way, further mistakes caused the core to heat to more than 9000 degrees Fahrenheit, producing molten metal that reacted with what cooling water was left to produce hydrogen gas and steam, resulting in a powerful explosion. What caused the second explosion is not clear, and experts disagree on what might have happened. Some theorize that it was a pure nuclear reaction.