In April 1986, the Chernobyl Nuclear Power Plant in Ukraine suffered a catastrophic reactor meltdown.
The resulting explosion and fire released large quantities of radioactive particles into the atmosphere.
It is the worst nuclear power plant accident in history in terms of cost and death toll.
The disaster began in Reactor 4 during a systems test, shortly after 1am on Saturday, 26 April 1986.
An unexpected power surge occurred during an experiment to test a new emergency core cooling feature.
Workers attempted an emergency shutdown, but this failed, causing an exponentially larger power surge.
According to some estimations, the reactor jumped to around ten times the normal operational output.
The reactor vessel ruptured and burning lumps of material and sparks shot into the air above the reactor.
Fuel escaping into the reactor's exterior cooling structure caused the destruction of the reactor casing.
The 2,000-ton upper plate of the reactor assembly was torn off by the explosion, sending it up through the roof.
The combustible graphite moderator was exposed to air, causing it to ignite.
The resulting fire sent a plume of highly radioactive fallout into the atmosphere and over a huge geographical area.
Thirty one deaths are directly attributed to the accident, all among the reactor staff and emergency workers.
Over 350,400 people have been evacuated and resettled from the most severely contaminated areas.
The accident raised concerns about the safety of the Soviet nuclear power industry, and nuclear power in general.
The disaster forced the Soviet government to become less secretive.
The government coverup was a catalyst for reforms leading to the Soviet collapse.
Russia, Ukraine, and Belarus have been burdened with the substantial decontamination and health care costs.
Estimates of the total number of deaths that will eventually result from the accident vary enormously.
The Chernobyl Forum predicts a death toll of 4,000 among those exposed to the highest levels of radiation.
In 1991, a report showed the reactors at Chernobyl didn’t comply with accepted standards of nuclear reactor safety.
An area originally extending 30km in all directions from the plant is officially called the "zone of alienation”.
It is largely uninhabited, except for about 300 residents who have refused to leave.
The Chernobyl reactor is now enclosed in a large concrete sarcophagus.
The sarcophagus was built quickly after the disaster, to allow the other reactors to remain operational.
Reactor 2 was shut down after a fire in 1992 and Reactor 1 was decommissioned in 1996.
Reactor 3 wasn’t turned off until 15 December 2000.
A new structure will be completed in 2016, the end of the 30-year lifespan of the original sarcophagus.
The structure is being built adjacent to the existing sarcophagus and will be slid into place on rails.
At 105m (344 ft) high and spanning 257 m (843 ft), it will cover both Reactor 4 and the hastily built 1986 structure.
Radiation levels are so high that builders only work 5 hours per day for one month before taking 15 days rest.
Ukrainian officials estimate the area will not be safe for human life again for another 20,000 years.
But if you can't wait that long, the sealed zone around Chernobyl was opened up to tourists in 2011.
Read more about the Chernobyl Disaster here.