The Chernobyl disaster of 1986 is the largest nuclear disaster in human history. It took place near the city of Pripyat in the former Soviet Union (now known as Ukraine) on the 26th of April.
During the Cold War the Soviet Union built many nuclear reactors in order to use the waste to build nuclear bombs.
The Chernobyl disaster caused large scale enviromental dammages. The dirt around the reactor can be fatal if you dig. Some body counts calculate that 38+ million people have died due to related cancers.
BP Deepwater Horizon Disaster
In 2010 one of British Petroleum's (BP) oil rigs safety systems failed causing a massive oil spill in the gulf of mexico. The equivalent of 4.9M barrels spilled into the ocean causing a massive enviromental disaster. Only 11 people were killed.