This past January, workers began construction on a new subway station in the heart of Los Angeles, California. They’re being more careful than usual as they dig beneath the city. Their machines could hit a mammoth!

Why are these ancient animals lurking beneath L.A.? About 40,000 years ago, during Earth’s last ice age, California was teeming with mammoths and other enormous mammals (see Ice Age Who’s Who). And some of these giants stuck around—literally.

Workers began building a new subway station this past January. It lies in the center of Los Angeles, California. The workers are digging under the city. And they’re being more careful than usual. Their machines could hit a mammoth!

Why are these ancient animals hiding under L.A.? California was once filled with mammoths and other huge mammals. They lived during Earth’s last ice age (see Ice Age Who’s Who). That was about 40,000 years ago. But the bodies of some of these giants stuck around. 

Los Angeles is dotted with sticky tar deposits that formed naturally over millions of years. The gooey pools trapped unlucky animals passing by. Everything from massive mammoths to tiny mice died and were preserved in the ancient sludge. 

Today, anyone who digs in the area has to be on the lookout. “It’s likely subway workers will not only find fossils, but potentially whole new tar pits containing tens of thousands of fossils,” says Emily Lindsey. She’s the lead paleontologist at the La Brea Tar Pits and Museum in downtown L.A. Scientists there are using the tar pits to piece together the city’s wild past. 

Los Angeles is dotted with tar pits. The sticky pools formed naturally over millions of years. They trapped unlucky animals passing by. Everything from big mammoths to tiny mice died and became preserved in the goo. 

Anyone who digs in the area has to be on the lookout today. “It’s likely subway workers will not only find fossils, but potentially whole new tar pits containing tens of thousands of fossils,” says Emily Lindsey. She works at the La Brea Tar Pits and Museum in downtown L.A. Lindsey is the lead paleontologist there. The museum’s scientists are using the tar pits to piece together the city’s wild past. 

Tar pits form very slowly. They start as natural oil fields deep underground. Over time, pressure from the rocks above forces some of the oil to the surface. The lighter parts of the oil evaporate, leaving behind pools of thick, sticky tar. 

Visitors to the La Brea Tar Pits can see—and smell—the large, muddy-looking tar pits around the property. They can also watch paleontologists dig up fossils. Scientists find huge extinct mammals, like giant sloths and mammoths. They also find animals that still roam California today, like badgers, snakes, and birds. 

Many of the preserved animals are predators. Dire wolves, an extinct species of wolf, are the most common. They were likely drawn to the area because they wanted to eat animals that were stuck in the tar. But when they went after their meal, they often got trapped too.

Tar pits form very slowly. They start as natural oil fields deep underground. The fields are under pressure from the rocks above. That forces some of the oil to the surface over time. The lighter parts of the oil evaporate. That leaves behind pools of thick tar. 

People can visit the La Brea Tar Pits. They can see and smell the muddy-looking tar. They can also watch paleontologists dig up fossils. Scientists find huge mammals, like giant sloths and mammoths. These animals no longer exist on Earth. Scientists also find animals that still live in California today. They include badgers, snakes, and birds. 

Many of the preserved animals were hunters. Dire wolves are the most common. They’re an ancient type of wolf. They likely came to the area to eat animals stuck in the tar. They went after their meal and often got trapped too.

Since 1875, paleontologists have unearthed more than 3.5 million fossils from the tar pits. Each find provides clues about California’s past. For example, scientists have uncovered fossils of plants that live only in cold or foggy areas. That means that L.A.’s climate was much cooler and wetter thousands of years ago.

At the new subway site, across the street from the museum, workers don’t want to damage any buried fossils. So the city hired monitors—people trained to look for fossils during construction. They watch the dirt as machines dig. 

“Most people probably wouldn’t notice a fossil. It’s the same color as the dirt,” says Ashley Leger. She’s the paleontologist overseeing the process. “Monitors know the texture and shape of bones,” she says. 

Construction workers take extra care too. On most projects, they use an excavator’s big claw to dig 1 meter (3 feet) deep with each swipe. At this site, they dig just 15 centimeters (6 inches) at a time. “If it weren’t for this process, the fossils would be in a million pieces,” says Leger. “We want them to be saved for people from all over the world to study.”

Paleontologists started studying the tar pits in 1875. They’ve dug up more than 3.5 million fossils since then. Each find gives clues about California’s past. For example, scientists have uncovered fossils of plants. The plants live only in cold or foggy areas. That means that L.A.’s climate has changed. It was much cooler and wetter thousands of years ago.

The new subway site sits across the street from the museum. Workers don’t want to harm any buried fossils there. So the city hired monitors. These people are trained to look for fossils during building. They watch the dirt as machines dig. 

“Most people probably wouldn’t notice a fossil. It’s the same color as the dirt,” says Ashley Leger. She’s a paleontologist. She’s heading the process. “Monitors know the texture and shape of bones,” she says. 

Workers take extra care too. They use an excavator. Usually, its big claw digs 1 meter (3 feet) deep with each swipe. But it digs just 15 centimeters (6 inches) at a time at this site. “If it weren’t for this process, the fossils would be in a million pieces,” says Leger. “We want them to be saved for people from all over the world to study.”

The more fossils workers find, the more scientists will learn about a time when ice age giants roamed the land. “It’s fascinating to sit in the tall buildings surrounding the tar pits and think about how this area once had mammoths wandering around,” says Leger.

Those mammoths may be long gone, but the tar pits are still active. New sticky pools bubble up all the time. Workers place bright-yellow cones around them to warn visitors to watch their step. 

“Things still get trapped in them—bugs, squirrels, and even trash,” says Lindsey. “It’s a modern-day record of life.”

Scientists learn more with each new fossil workers find. The fossils tell a story about a time when ice age giants walked the land. “It’s fascinating to sit in the tall buildings surrounding the tar pits and think about how this area once had mammoths wandering around,” says Leger.

Those mammoths are long gone. But the tar pits are still active. New sticky pools bubble up all the time. Workers place yellow cones around them. They warn visitors to watch their step. 

“Things still get trapped in them—bugs, squirrels, and even trash,” says Lindsey. “It’s a modern-day record of life.”