Around 1850, Eduard Suess was the first to claim that the Earth's crust was the same at the bottom of oceans as it is on the tops of mountains, and used fossils to prove that mountains could not have emerged all at once. In the early 20th century, Alfred Wegener continued Suess's work and noticed that the continental coastlines fit together perfectly, and imagined they had once been a single continent, which he called Pangea. Around the same time, Arthur Holmes managed to date rocks and determined the earth's age. Thanks to sonar technology from WWII, accurate maps of the ocean floor allowed Mary Tharp was able to reveal the extent of mid ocean ridges and oceanic trenches. In the 1960s, scientists managed to sample the ocean floor and found Basalt rock, which meant the ocean was prone to seismic activity. In 1960, Harry Hammond Hess theorized that the oceans were like a conveyer belt moving the moving the continents around. Later, Tuzo Wilson, Jason Morgan, and Xavier Le Pichon discovered that the earth's surface is divided into six main plates. Their movement tied to the shifting of the continents is called plate tectonics.