In 1902, Marie and Pierre Curie isolated the radioactive element radium for the first time.
In 1946, the League of Nations officially dissolved.
In 1999, two gun-wielding students killed 12 fellow students and a teacher at Columbine High School in Littleton, Colo.
In 2010, a gas explosion and fire killed 11 people on the Deepwater Horizon oil rig in the Gulf of Mexico, beginning an oil spill that wou…