Approximately _____________ago, the earth had dense forests in low-lying wetland areas in which coal was formed.

The formation of coal started approximately 300 million years ago.

Approximately 300 million years ago, the earth had dense forests in low-lying wetland areas. Due to natural processes, like floods, these forests got buried under the soil. As more soil deposited over them, they were compressed. The temperature also rose as they sank deeper and deeper. Due to high pressure and temperature, coal was formed.