In April 1986, a crew at the Chernobyl nuclear power plant in Ukraine —then part of the Soviet Union — ran a seemingly routine test to see how long a reactor's turbines would continue to supply power to its circulating pumps in the event of a loss of electrical power. The reactor malfunctioned due to an inopportune power surge, and the fuel rods got stuck, overheating the water inside the reactor and causing a buildup of steam. The resulting explosions caused massive amounts of radioactive gases and debris to spew into the atmosphere for 10 days — the biggest such uncontrolled release in history not from a nuclear bomb.
Two workers died immediately from the explosion. Twenty-eight more, including six firemen who struggled to put out fires on one of the plant's rooftops, died later from radiation exposure, and winds carried the radiation far and wide across the Soviet Union and even to other European countries [source: World Nuclear Association]. But despite the magnitude of the disaster, Soviet officials didn't publicly admit that the accident had occurred until two days later, when Swedish officials sounded the alarm about increased levels of radiation drifting westward.
Then-Soviet leader Mikhail Gorbachev waited an astonishing three weeks before even mentioning the accident publicly. He later claimed, somewhat implausibly, that the Kremlin had difficulty getting the full story, and "we realized the entire drama only later." But the rest of the world responded with such scathing criticism that Gorbachev felt compelled to lift information restrictions, not just about the disaster but other government misdeeds as well. That period of "glasnost," or openness, ultimately hastened the end of the Soviet regime itself a few years later [source: Associated Press].