Kaali is a group of 9 meteorite craters located in the village of Kaali on the Estonian island of Saaremaa. Formed more than 4,000 years ago (the age of the sediments at the bottom of crater lake), and possibly as long as 7,600 years ago (estimates vary), it is a comparatively recent crater. It was created by an impact event and is possibly the only known major impact event that has occurred in a populated area.
Prior to the 1930s there were several hypothesis about the origin of the crater, including theories involving vulcanism and karst processes. Its meteoritic origins were first conclusively demonstrated by Ivan Reinvald in 1928, 1933 and 1937.