During a late-night safety test, operators at the Chernobyl plant in Soviet Ukraine lost control of Reactor No. 4, triggering a steam explosion and fire that blew off the reactor's roof. The disaster killed dozens outright, forced the evacuation of over 100,000 people, and spread radioactive contamination across much of Europe, becoming the worst nuclear accident in history.
At 1:23 a.m., engineers ran a test meant to see whether turbines could power emergency cooling pumps during a power failure. Instead, a combination of reactor design flaws and operator error caused power to surge uncontrollably. Two explosions tore through the reactor building, exposing the core to open air and igniting a graphite fire that burned for days, pumping radioactive smoke into the atmosphere.
Soviet authorities initially said nothing. The nearby city of Pripyat, home to 50,000 workers and families, wasn't evacuated until 36 hours later, and by then many residents had already absorbed dangerous doses of radiation. Firefighters and plant workers who battled the blaze in the first hours, some without knowing the danger, suffered fatal radiation sickness within weeks.
The accident forced the abandonment of a 1,000-square-mile exclusion zone that remains largely uninhabited today, wolves and lynx now roaming the empty streets of Pripyat. Chernobyl accelerated the decline of Soviet credibility, with Gorbachev later calling it a turning point that hastened the USSR's collapse more than any single event.