Liquid Layer.

In 1960 the theory of plate tectonics was given a huge boost by the oceanographer Harry H. Hess, who demonstrated his theory that the seafloor wasspreading. The Earth is composed of layers — the outer crust, beneath that the mantle, and below that the core. Hess suggested that the mantle, which is eighteen hundred miles thick, has two layers. The deeper layer is solid, he said, which accorded with generally accepted theories about the mantle. But the upper mantle was more like a hot liquid, according to Hess. It pushed up from below. In the deep seafloor rifts, the mantle is close to the surface. It therefore actually comes to the surface (the seabed) and pushes the Earth's crust aside on either end of the rift.

Mohorovicic Discontinuity.

The easiest way to prove Hess's theory was to drill a hole in the seafloor...