41960 Laobaidong Coalmine accident (China, 684 casualties)
On May 9, 1960, the Laobaidong Coalmine experienced a methane gas explosion that killed 684 people. This disaster is the second worst disaster in Chinas history. Atop of that the disaster was kept secret for about three decades as the Chinese Government sought to hide the accident as best it could. Only in 1992 did the Chinese Government fully explain the details behind the Laobaidong coalmine.