Harry Hammond Hess, who during World War II served in the United States Navy Reserve and became fascinated in the ocean floor, studied the Mid-Atlantic Ridge and proposed that hot molten rock was added to the crust at the ridge and expanded the seafloor outward. This theory was to become known as seafloor spreading. Since the Earth's circumference has not changed over geologic time, Hess concluded that older seafloor has to be consumed somewhere else, and suggested that this process takes place at oceanic trenches, where the crust would be melted and recycled in the Earth's mantle.