Shale is a sedimentary rock laid down as mud at the bottom of the seas and lakes and compressed over hundreds of millions of year until it hardens into rock. All sediments contain dead organic material such as algae. Under extreme pressure and temperature this converts to oil and gas. Some is trapped. With sandstone, another sedimentary rock, the grains, and thus the space between them, is greater than shale. Oil and gas will move naturally, if slowly, through sandstone and can be pumped out. A technique has now been developed to get gas from the much tighter shale formations. The reserves of shale gas are huge and, equally important, many occur in America and Europe.