When Ashley Leger’s phone buzzes, she pulls on her neon vest, hard hat, and goggles. She climbs deep down into a huge construction site under a road in Los Angeles, California. Ms. Leger isn’t a construction worker. She’s a paleontologist—a scientist who studies fossils.

Ms. Leger gets on her hands and knees. She gently brushes dirt from a spot pointed out by a member of her team. Her heart starts to beat fast. Is she about to dig up something important?

Many paleontologists dig into the ground in open plains or deserts. But Ms. Leger works in the middle of a city. Did you know that paleontologists often work for the city of Los Angeles? They have for a long time. Subway diggers burrow deep into the ground to make modern tunnels for speeding trains. But they never know when they’ll run into ancient history. When they find an underground artifact that’s big enough to need digging out, they call a paleontologist like Ms. Leger.

Workers started expanding the subway on the west side of Los Angeles in 2014. Since then, they’ve found part of a rabbit jaw, a mastodon tooth, a camel leg, bison vertebrae, and a tooth and ankle bone from a horse.

Ms. Leger thinks about those discoveries as she bends down and begins to dig. She’s approaching something a lot bigger than a jaw or even a leg. It looks like an elephant skull.

It takes 15 hours to dig out the skull. The skull weighs a few hundred pounds. It’s as big as an easy chair! Soon something becomes clear: This skull didn’t belong to an elephant. Two tusks poke out of it! It belonged to the elephant’s ancient relative—a mammoth.

Ms. Leger stands up and dusts herself off. She’s had more than a good day’s work. She has made the find of a lifetime!