Earth's mantle is the 2,900 km thick rocky shell making up about 84% of Earth's volume. It is mostly solid and lies over the Earth's iron-rich core, which takes up about 15% of Earth's volume. Past episodes of melting and volcanism at the outer levels of the mantle have produced a very thin crust of crystallized melt products near the surface, where we live. The gases evolved during the melting of Earth's mantle have a large effect on the composition and size of Earth's atmosphere.

Earth cutaway from core to exosphere.

On March 5, 2007, a team of scientists on board the RRS James Cook went on a voyage to an area of the Atlantic seafloor where the mantle has no crust covering. The anomaly is located midway between the Cape Verdes Islands and the Caribbean in the Atlantic Ocean. It lies about three kilometres under the ocean surface and covers thousands of square kilometres.