The Pit 91 excavation site is located about 100 yards west of the La Brea Tar Pits Museum in Hancock Park. It was the ninety-first pit in a sequence of 96 sites explored during the 1913-1915 excavations undertaken by the Los Angeles County Museum. Digging began on June 13, 1915 and by July, after reaching a depth of approximately nine feet, it was decided that this large deposit of fossils would be left in situ as a "showpiece" for visitors. It subsequently suffered repeated cave-ins and floods, and was abandoned with thousands of fossils still awaiting excavation.
On June 13, 1969 (or "Asphalt Friday" as it has come to be known), Pit 91 was reopened with the idea of collecting everything; not only just large vertebrate fossils but also the smaller fossils that had been ignored by early excavators. Processing of the matrix surrounding the larger fossils has allowed us to recover microfossils including shells, plants, insects, fish, amphibians, reptiles, birds, and small mammals. These in turn have provided us with valuable environmental information about Los Angeles Basin in the late Pleistocene. Watch footage from the opening ceremony on "Asphalt Friday"
Excavation continued year-round in Pit 91 until 1980, when the site was closed due to budget constraints. Then in 1984, for the Summer Olympic Games, it was reopened and between then and 2007, excavation took place for 10 to 12 weeks every summer. The excavators returned again in the summer of 2014.
In the 2007 field season, 3,388 specimens were recovered from Pit 91, including:
- two saber-toothed cat skulls,
- six dire wolf skulls,
- a near-complete horse skull,
- several Harlan's ground sloth limb bones,
- a juvenile Shasta ground sloth jaw,
- the first confirmed piece of a (very young) mammoth from Pit 91.
Pit 91 is currently 15 feet deep; it is estimated that the deposit extends another 3 to 8 feet further below ground.