Wikipedia:Selected anniversaries/April 22
- 1500 – Portuguese explorer Pedro Álvares Cabral (pictured) and his crew became the first Europeans to sight Brazil when they spotted Monte Pascoal.
- 1889 – Over 50,000 people rushed to claim a piece of the available two million acres (8,000 km²) in the Unassigned Lands, the present-day U.S. state of Oklahoma. Within hours, both Oklahoma City and Guthrie had established cities of around 10,000 people.
- 1915 – The Germans released chlorine gas as a chemical weapon in the Second Battle of Ypres, killing over 5,000 soldiers within ten minutes by asphyxiation in the first large-scale successful use of poison gas in World War I.
- 1930 – France, Italy, Japan, the United Kingdom, and the United States signed the London Naval Treaty, regulating submarine warfare and limiting military ship building.
- 1945 – About 600 prisoners of the Jasenovac concentration camp in the Independent State of Croatia revolted, but only 80 managed to escape while the other 520 were killed by the Croatian Ustaše regime.
- 1993 – The first version of Mosaic, created by computer programmers Marc Andreessen and Eric Bina at the National Center for Supercomputing Applications of the University of Illinois at Urbana-Champaign, was released, becoming the first popular World Wide Web browser and Gopher client.