Natural gas was formed from the remains of plants and animals around one hundred million years ago. The process started much earlier, about three hundred million years ago, when the Netherlands was made up of marshy lowlands near the equator with a very hot and humid tropical climate. At this time, there was a flourishing animal and plant life, including giant insects, amphibians, huge trees and ferns. When the trees and plants died, they rotted down in the swampy land. This resulted in a thick layer of peat, in which the remains of animals were also preserved. From time to time, the sea came right up and washed over the low-lying land, leaving clay or sand on the peat. These deposits and the peat formations created alternate layers. There was increasing pressure on the peat, which eventually changed into coal and became hotter. This process caused natural gas to be released. At first, the clay layers prevented the lightweight gas from escaping. Later, when stresses in the earth’s crust caused cavities to appear, some pockets of gas were blocked from escaping by a salt crust created by sea water. Gas fields have been created in those places where a salt layer has obstructed the rising gas from escaping.