What is coal? http://www.nationalgeographic.org/encyclopedia/coal/ https://www.nationalgeographic.org/encyclopedia/coal/ coal types https://www.nationalgeographic.org/encyclopedia/coal/ https://www.nationalgeographic.org/encyclopedia/coal/ 300 million years ago types https://www.nationalgeographic.org/encyclopedia/coal/ http://www.nationalgeographic.org/encyclopedia/coal/ https://www.nationalgeographic.org/encyclopedia/coal/ http://www.nationalgeographic.org/encyclopedia/coal/