June 30th

The remains of a Vietnam War serviceman buried in the Tomb of the Unknown Soldiers were identified as those of Air Force pilot Michael J. Blassie.
Tomb of the Unknowns
South African ANC President Nelson Mandela meets with UN Secretary-General Boutros Boutros-Ghali at Dakar.
Richard Goldstone
The first leap second is added to UTC. Leap seconds are added to Coordinated Universal Time (UTC) from time to time to account for the Earth's slowing rotation. UTC is the basis for the calculation of local times worldwide.
Leap second
The crew of Soviet spacecraft “Soyuz 11” dies after the loss of air supply. The cause of the tragedy was a faulty valve. The three cosmonauts had previously achieved the first docking of a spacecraft to a space station in history.
Soyuz 11
Superman 1st appears in DC Comics' Action Comics Series issue #1.
Action Comics 1
The novel “Gone with the Wind” is published. Margaret Mitchell's story set in the American South during the American Civil War became one of the United States' biggest best-sellers. The 1939 movie version starring Vivien Leigh and Clark Gable was similarly successful.
Gone with the Wind (novel)
Adolf Hitler secured his position in the Nazi party by a "blood purge," ridding the party of other leaders such as Ernst Roehm and Kurt von Schleicher.
President Warren G. Harding appointed former president William H. Taft chief justice of the United States.
President William Howard Taft
The Tunguska event leaves 2000 square km of Siberian forest flattened and scorched. The devastation is believed to have been caused by the explosion of an asteroid or comet. It is the largest impact event in recorded history.
Albert Einstein submits a paper outlining his theory of special relativity. The text “Zur Elektrodynamik bewegter Körper” comprises the currently accepted theory about the relationship between space and time. The theory of special relativity is the basis for his theory of general relativity, which he published in 1916.
List of scientific publications by Albert Einstein
Famous debate on Charles Darwin's theory of evolution held at the Oxford University Museum and dominated by arguments between Thomas Henry Huxley and Bishop Samuel Wilberforce.
French acrobat Charles Blondin, AKA Jean Francois Gravelet, walked across Niagara Falls on a tightrope.
Charles Blondin
The Spaniards are expelled from Tenochtitlan.
Fall of Tenochtitlan