Floating on the Earth's core is the ... a soft layer about 1,800 miles thick of molten rock.

what is it?

Mantle

Explanation:

The mantle is the mostly-solid bulk of Earth's interior. The mantle lies between Earth's dense, super-heated core and its thin outer layer, the crust. The mantle is about 2,900 kilometers (1,802 miles) thick, and makes up a whopping 84% of Earth's total volume.