In 1912 a German scientist named Alfred Wegener suggested something that most geologists of his time thought was impossible: the continents move. Wegener had trained in astronomy and worked as a meteorologist, so he was an outsider to geology. He set out his idea, which he called continental drift, in a book published in 1915, The Origin of Continents and Oceans.

Wegener gathered evidence from several different fields. The coastlines of South America and Africa look as though they would fit together. Fossils of Mesosaurus, a small freshwater reptile that could not have swum across an ocean, are found in both South America and southern Africa. Fossils of the plant Glossopteris turn up in South America, Africa, India, Australia and Antarctica. Rock layers and mountain belts that stop at one coastline continue on another continent. Scratches left by ancient glaciers appear in places that are warm today. Wegener argued that all this made sense if the continents had once been joined in a single supercontinent, which he called Pangaea.

His evidence was strong, but his explanation was not. Wegener suggested the continents ploughed through the ocean floor, and physicists showed that the forces he proposed were far too weak to do that. Without a believable mechanism, most geologists rejected continental drift, and Wegener died in 1930 during an expedition on the Greenland ice sheet without seeing his idea accepted.

The evidence that changed their minds came from the sea floor. After the Second World War, new equipment mapped long mountain ranges running down the middle of the oceans. In 1962 Harry Hess proposed sea-floor spreading: new sea floor forms at these mid-ocean ridges and moves outwards. In 1963 Fred Vine and Drummond Matthews showed that magnetic stripes recorded in the sea floor are symmetrical on both sides of a ridge, matching known reversals of Earth's magnetic field. That was the missing mechanism. By the late 1960s the theory of plate tectonics was accepted, and Wegener's central claim was shown to be right, even though his explanation of how it happened was wrong.