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.
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 mid-way 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.