The fossilized remains of creatures that roamed southern California 10,000 years ago were discovered by a crew digging for a subway extension under the busy streets of Los Angeles.

Ashley Leger works for a company contracted by Los Angeles transportation officials to keep paleontologists on hand as the city extends the purple line to the west side.

When Leger gets a notification on her phone, she dons a neon vest, hard hat and goggles before climbing deep into a massive construction site beneath a boulevard east of downtown.

Since work on the extension began in 2014, fossilized remains—including a partial rabbit jaw, mastodon tooth, camel foreleg, bison vertebrae and a horse's ankle bone—have turned up from creatures that roamed the grasslands and forests that covered the region about 10,000 years ago in the last Ice Age.

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But the discovery that makes Leger shake her head in disbelief came about a year ago. When she arrived at the site, Leger recognized what appeared to be a partial elephant skull.

It turned out to be a much bigger discovery. After 15 hours of excavation, the team uncovered an intact skull of a juvenile mammoth.

“It's an absolute dream come true for me,” Leger, who spent the previous decade at a South Dakota mammoth site with no discoveries even close to the size of the one in Los Angeles, said. “It's the one fossil you always want to find in your career.”

California's strict environmental laws require scientists to be on hand at certain construction sites.

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Assistant curator Dr. Emily Lindsey called it a "pretty remarkable find," noting that while thousands of dire wolf and saber-toothed cat remains have been uncovered in L.A., there have been only about 30 mammoths.

A few hundred pounds, the skull is especially rare because both tusks were attached. It's being studied and is available for public viewing inside the museum's glass-walled Fossil Lab.

With a nod to Hollywood, the 8- to 12-year-old Colombian mammoth was named Hayden, for the actress Hayden Panettiere, featured in the TV series "Nashville" and "Heroes."

The Associated Press contributed to this report.