The Messel Pit was originally a disused oil shale mine that gained fame in the early 20th century for its wealth of fossils. Mining was highly active here from 1859 to 1971. Later, authorities planned to use the pit as a landfill for waste disposal—a dispute that persisted until 1991, when the government of Hesse (the state where the site is located) purchased the property for long-term scientific research. On June 14 of that same year, it was designated a protected heritage site. Situated in Hesse, south of Frankfurt, Germany, its fossils provide invaluable, unique insights into early mammalian evolution. Commercial and private fossil excavations did not last long, however; they ceased following the site's inscription as a UNESCO World Heritage site in 1995. As a result, specimens circulating on the market today are almost exclusively older collection pieces excavated between the 1970s and 1995. Messel oil shale contains roughly 15% kerogen and 40% water. Once excavated, the shale dries out and shatters as the moisture evaporates. To protect these delicate fossil specimens from destruction, preparators employ a meticulous transfer technique using synthetic resin to embed and seal the fossils before they dry out—which is why the matrix on Messel specimens often looks somewhat artificial.