The McKittrick Tar Pits in southern California are the state's largest natural asphalt lake area and one of only five such areas known worldwide. Formed probably during the Pleistocene as oil rose along deep faults from the Monterey Formation, their seeps thickened into bitumen and asphalt. Yokuts people long used the asphalt for waterproofing, decoration, and trade, and settlers began commercial extraction in the 1860s. The pits also became a major paleontological site because asphalt preserved Pleistocene animals, including lions, saber-toothed cats, mammoths, birds, and insects. Excavations in the 20th century documented dozens of mammal and bird species, confirming the site's geological, human, and scientific importance.