Snowball Earth is a long debated and controversial theory that a thick sheet of ice covers Earth in its entirety about 650 million years ago. The theory was first formed in the 1940s by Brian Harland after he learned of the existence of dropstones in the Arctic, and later discover that there are dropstones all around the world, even on the hottest continents and in the tropics. Harland hypothesized that ice spread to every continent with continental drift explaining the spread of dropstones, but this was swiftly rejected by scientists that viewed it as nonsensical and impossible.
Russian scientist, Mikhail Budyko, later formed his own theory of Snowball Earth which was that the tropics could not freeze in practice, but could happen in theory.