Which layer of the Earth is liquid?

The outer core of the Earth is the layer that is liquid. This layer is primarily composed of molten iron and nickel, and it lies beneath the mantle and above the inner core. The outer core's liquid state is a result of extremely high temperatures that surpass the melting point of iron at the substantial pressures found at that depth. This fluid movement within the outer core is crucial because it generates the Earth's magnetic field through the dynamo effect.

In contrast, the inner core, while extremely hot, remains solid due to the immense pressures that exist at that depth. The crust is the Earth's outermost layer, which is solid, and the mantle, while it can behave like a viscous fluid over long periods, is primarily solid with some semi-molten regions known as the asthenosphere. The unique characteristics of the outer core set it apart as the only liquid layer of the Earth's interior.