The next layer down is the mantle. It reaches a depth of about 1,800 miles (2,900 kilometers) and is also made of solid rock. But there is great heat and pressure at these depths, so the mantle is slightly flexible. The innermost part of Earth is the core, which is about 4,200 miles (6,700 kilometers) across and consists mostly of iron. The inner core is solid, while the outer core is a liquid.