The main substance that makes up the Earth's core is iron. The Earth's core consists of two layers: a solid inner core and a liquid outer core. The inner core is composed predominantly of iron and nickel, while the outer core is mainly molten iron and other lighter elements. The immense pressure and temperature in the core influence its physical state. This composition is responsible for generating the Earth's magnetic field through the dynamo effect caused by the movement of the molten outer core. The core plays a critical role in our planet's geology and magnetic properties, influencing many Earth processes.