More than 99 percent of diamonds ever mined have formed in the conditions of extreme heat and pressure about 140 kilometres (87 mi) below the earth's surface. Coal is formed from prehistoric plants buried much closer to the surface, and is unlikely to migrate below 3.2 kilometres (2.0 mi) through common geological processes. Most diamonds that have been dated are older than the first land plants, and are therefore older than coal. It is possible that diamonds can form from coal in subduction zones and in meteoroid impacts, but diamonds formed in this way are rare and the carbon source is more likely carbonate rocks and organic carbon in sediments, rather than coal.