 This appeared to be confirmed by the exploration of the deep sea beds conducted by the Challenger expedition, 1872–1876, which showed that contrary to expectation, land debris brought down by rivers to the ocean is deposited comparatively close to the shore on what is now known as the continental shelf.This suggested that the oceans were a permanent feature of the Earth's surface, and did not change places with the continents.Apart from the earlier speculations mentioned in the previous section, the idea that the American continents had once formed a single landmass together with Europe and Asia before assuming their present shapes and positions was postulated by several scientists before Alfred Wegener's 1912 paper.