The continents are fixed in place and have always occupied their current positions.

The solid ground beneath your feet seems like the most permanent thing imaginable. Mountains, coastlines, continents, these were the stable backdrop against which human history played out. Geology textbooks through the mid-20th century described a fixed Earth: continents had always been where they are, oceans had always been where they are, and the great landmasses moved only in the sense that rivers eroded them and volcanoes built them up. The stability of continents was not a naive assumption, it was the working hypothesis of a discipline that had spent a century mapping and classifying rocks.

Alfred Wegener disagreed, loudly, starting in 1912. The German meteorologist had noticed what any careful look at a map reveals: South America's eastern coast and Africa's western coast fit together like puzzle pieces. Fossil records confirmed it, identical species of land animals and plants on continents now separated by thousands of miles of open ocean. Wegener proposed they had once been joined in a supercontinent he called Pangaea, and had since drifted apart. The geological establishment was scornful. Without a credible mechanism, some force capable of moving entire landmasses, the theory was dismissed as speculation from an outsider discipline.

The mechanism arrived from the ocean floor. During and after World War II, the US Navy mapped the seabed using sonar with unprecedented resolution. The results were extraordinary: running through every ocean was a continuous underwater mountain range, the mid-ocean ridge system, with young volcanic rock at its centre and progressively older rock toward the continents. Harry Hess, a Princeton geologist and Navy reserve officer, synthesised the data in 1960 into the seafloor spreading hypothesis: magma wells up at mid-ocean ridges, solidifies into new oceanic crust, and pushes the seafloor, and the continents riding atop it, steadily outward.

The confirmation came from magnetism. As molten rock hardens at the ridges, it freezes the orientation of Earth's magnetic field at that moment. Because Earth's field reverses periodically, the spreading seafloor should preserve a symmetrical zebra-stripe pattern of magnetic polarity on either side of each ridge, if spreading was real. In 1963, Fred Vine and Drummond Matthews at Cambridge found exactly that pattern in the North Atlantic. The theory of plate tectonics, integrating continental drift, seafloor spreading, and subduction, became the unifying framework of all Earth sciences within a decade.

The continents are moving right now, at roughly the rate a fingernail grows, about 2.5 centimetres per year. North America and Europe are pulling apart; India is still slowly driving into Asia, pushing the Himalayas slightly higher every year. In 250 million years, the Atlantic will have widened, the Pacific narrowed, and the continents may have reassembled into a new Pangaea. The permanence of the ground underfoot is an illusion on geological time.