Coal in all its forms derives from the remains of plants that grew on Earth about 250 million years ago. These plants decayed and were eventually turned into coal by the intense heat and pressure of the Earth’s geological forces. Since its discovery in prehistoric times as a mineral that burns easily, it has been increasingly used to generate heat. Although its use globally has been declining since 2013, thermal coal still supplies about a quarter of the world’s primary energy and two-fifths of its electricity, according to the International Energy Agency (IEA).