Millions of years ago, a large number of organisms perished and their remains fell to the bottom of the ocean. Over time, they became covered in sediment and were exposed to intense heat and pressure, which transformed them into what we know as crude oil, gas and coal. These are also known as fossil fuels. This fossilized organic matter is a large carbon store.
Fossil fuels are non-renewable resources, as humans are burning them far quicker than they can be replenished. Burning fossil fuels releases greenhouse gases, excessive burning can lead to global warming.