Harry Hammond Hess was a professor who believed many of Alfred Wegener's points in his theory but he had very different opinions about how the plate tectonics moved. He came up with the brilliant hypothesis of seafloor spreading. Seafloor spreading is when magma from the upper mantle flows up onto the seafloor. The magma then covers the deep ocean trenches, creating new seafloor. Hess explained all of this in his book, "History of Ocean Basins" which was a very big contribution to the development of plate tectonics. Harry came up with his hypothesis while fighting in WW2 in the ocean. He was able to do some echo- sounding surveys in the Pacific Ocean. Hess' hypothesis was accepted by the scientific community after other scientists did some tests.