This abyss is tucked between world-class art museums, high-rise condominiums and some of the busiest automobile traffic in the United States. The site is even an unknown to many city residents.

But since 1969, scientists have mined this urban bone yard and built a massive collection of pre-history, with some bones dating back 40,000 years.

"This is the richest Ice Age deposit in the world," chief curator of the Tar Pits, John Harris told AFP. "We will still be pulling out bones in 20 years."

Last summer scientists removed some 3,000 bones from the site, from the jawbones of mice, to the leg bones of mastodons. The site was first discovered in 1915 during construction of the Los Angeles County Museum of Art.

The collections now approach four million items from 12 deposits spread over several acres, including more than 650 species of plants and animals.

"Tip the world over on its side and everything loose will land in Los Angeles," US architect, Frank Lloyd Wright once joked.

Thousands of ancient mammals were just passing through the prehistoric Los Angeles basin when they were trapped in the bubbling black ooze formed by the petroleum deposit. Much like the city of Los Angeles, the tar pit contains more transplants than it does natives.

The richness of the pit is staggering even to the excavators.

"Everywhere you look there's bones," said former anatomy professor and volunteer excavator, Jean Moore.

"In the twelve years that I've been digging in this hole, I've probably gone through 5,000 years of natural history," she said while picking at the pelvis of a prehistoric cat with tar caked fingernails.

With the issue of global warming entering mainstream dialogue, paleontologists believe their work at La Brea may become even more relevant.

"The more we know about prehistoric climate change, the more we'll know about what's going on right now," said chief excavator Kristen Vowells.

"We're looking back to look forward," she said while logging thumbnail sized finds and securing them in small envelopes.

The Tar Pits' research staff has such a backlog of fossils and bones that diggers could take a season or two off and cataloguers would still have more than enough work.

"We're absolutely running out of space," laboratory supervisor Shelley Cox told AFP surrounded by floor-to-ceiling stacks of various relics marked "waiting to be cleaned" and "waiting to be catalogued".

"But if we find something really cool, we'll bump it to the front of the line," she said.

Last summer, diggers discovered the skull of a 30,000 year-old saber-toothed cat with its jaws in place. "He stepped into the tar in the prime of his life," said Cox while pointing at the teeth.

"We named him Max."

The tar deposits are believed to spread throughout the immediate area. The neighborhood adjacent to the pits recently spent 10 million dollars to resurface and landscape the streets, according to Harris, and within a year the tar bubbled back up through the streets and sidewalks.

It continues to trap the occasional squirrel or loose house cat wandering into the muck.

Exciting finds often coincide with local development. The Los Angeles County Museum of Art, the pit's immediate neighbor to the south, recently began construction of a parking facility.

In the processing of digging, they found the entire skeleton of a mastodon, the size of a city bus.

Cox said a planned subterranean metro line down Wilshire Boulevard, known locally as Miracle Mile, just south of the pits, will turn up a whole new collection of deposits.

"As the city continues to develop, we're going to have a major storage problem," said Cox.

But the lack of space does not dampen her enthusiasm for her place as recorder of the natural history of Los Angeles. Though thousands pass the site unaware, paleontologists world wide rely on La Brea's collection for comparative study.

"We are the mecca for Ice Age mammals," said Cox.

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