Scientists are unearthing traces of the distant past at the Rancho La Brea Tar Pits in Los Angeles, one of the world's most famous fossil locations. The annual summertime excavation at Pit 91 is yielding clues about ancient animal and plant life.

Today, the tar pit is a public park and a popular destination for schoolchildren.

But 15,000 years ago, giant mammoths, mastodons, saber-tooth cats and dire wolves roamed here. The fossilized remains of thousands have been found in the tar pits.

John Harris is chief curator of the Page Museum at the Rancho La Brea Tar Pits. He says there are similar sites around the world, but this site is special.

"There are two more in California," said John Harris. "There are some in South America and some in Europe and Asia, but none of them have either the volume of fossils or the diversity of fossils that we've recovered here from Rancho La Brea."

Staff workers and volunteers dig in Pit 91 for 13 weeks each summer, carefully extracting fossilized bones, mollusks, seeds and twigs from the tar, which has oozed out of the ground for thousands of years, trapping animals that set foot in the sticky substance.

The farther down researchers dig, the further into the past they are traveling.

Pit 91 tells the story of 40,000 years of natural history. Excavation coordinator Christopher Shaw says the work is dirty, but exciting.

"And it's like the treasure-seekers, you know, it is the thrill of discovery, and then figuring out exactly what it is that you've found," said Christopher Shaw.

That work is done in the museum's laboratory. Supervisor Shelley Cox says workers carefully remove sediment from the fossilized remains of the animals. The sediment contains fossilized seeds, snail shells and rodent bones.

"We want that sediment right up against the bone for further analysis, to see what little fossils got preserved along with the big bone," said Shelley Cox.

Scientists say some 12,000 years ago, many large mammals, including mastodons, mammoths and saber-tooth cats disappeared.

Carbon dioxide in the atmosphere had been reduced during the Ice Age, a few thousand years earlier. That decreased the number of plants that the animals used as food. Then humans entered the New World, hunting the big mammals and competing with them for resources.

Today, at the La Brea Tar Pits, visitors are hearing the story, and getting a glimpse of this long-lost age.