Fossils form rarely, since an organism is usually consumed totally or scattered by scavengers after death. If the structures remain intact, fossils can be preserved in amber (hardened tree sap), Siberian permafrost, dry caves, or rock. Rock fossils are the most common. In order to form a rock fossil, three things must happen: 1) the organism must be buried in sediment; 2) the hard structures must mineralize; and 3) the sediment surrounding the fossil must harden and become rock. Many rock fossils either erode before they can be discovered, or they remain in places inaccessible to scientists.