Los Angeles is the Tar Pits

According to the Page Museum:

A large petroleum reservoir called the Salt Lake Oil Field is located below the surface a short distance to the north of Hancock Park. The oil was formed from marine plankton deposited in an ocean basin during the Miocene Epoch (5-25 million years ago). Time and pressure converted the organisms into oil. For about 40, 000 years, this petroleum has been migrating to the surface, either along a faulted sedimentary zone or along steeply dipping, porous sedimentary rock layers.

Although large quantities of asphalt comes up in the former excavation pits, visitors can also observe asphalt seeping onto the surface of the ground outside the fenced areas. In fact, asphalt seepage is not restricted to Hancock Park. For several blocks in all directions, asphalt has been found seeping onto surface streets, into sewers and under buildings. The bubbles seen in the Lake Pit and at other pit sites are composed of methane, commonly called as natural gas, escaping into the atmosphere. Methane gas, which is colorless and odorless, is the same substance used in gas-burning home appliances (the gas company adds an odor for safety reasons). Methane is a by-product created when plant and animal remains decompose with the formation of crude oil. The "rotten egg" odor is hydrogen sulfide, another by-product of decomposition.