As fears mount over a meltdown in Japan's earthquake-hit Fukushima nuclear reactor, experts refer to a previous nuclear disaster.
On April 26, 1986 a Soviet power plant in Chernobyl in what is now Ukraine suffered history's worst nuclear disaster. A blast ripped through a reactor and the resulting fire sent plumes of radioactive fallout throughout most of Europe.
The hardest hit was the western part of the Soviet Union, including Ukraine, Belarus and Russia.
Hundreds of thousands of people needed to be resettled and there was permanent damage to the environment. The nearby town of Pripyat became a ghost town.
Many people near the site suffered acute radiation sickness. Dozens died within months. Thousands of cancer deaths in later years were attributed to the fallout from Chernobyl.
A massive concrete structure was built around the damaged reactor to contain radiation leakage. A new steel shelter to replace it will be completed next year.
The disaster in Chernobyl continues to serve as a warning about the dangers of nuclear power.
Initially, Soviet authorities tried to cover up the extent of the disaster. Some people are concerned that Japanese authorities might now be doing the same.