In the La Brea tar pits, smack in the middle of Los Angeles, paleontologists have unearthed gigantic mastodons, saber toothed cats and other ice age treasures that were trapped in the tar. The tar pits were, and are, a deceptively dangerous place, says Earth magazine. “As little as four centimeters of tar could be enough to ensnare a large animal.”

Unlike most fossil quarries, the La Brea tar pits are still an active hazard. “Working at the tar pits, at some point you’re going to step in a tar seep. It’s almost a rite of passage,” says Anna Holden, a paleoentomologist at the Natural History Museum of Los Angeles County in California.

Once stuck in a tar seep, animals would eventually sink into the earth. But that’s not the only reason they were deadly, says Earth, describing new research. Once ensnared, animals would linger on the surface for months—often 17 to 20 weeks. Stuck there, they were tempting bait to roaming scavengers.

Dire wolves, which roamed the western U.S. until 11,000 years ago, were often tricked by what seemed like an easy meal, says the Page Museum, which works with fossils from the tar pits. More than 4,000 dire wolves have been pulled from the pits, the museum says: “Most were probably trapped while attempting to feed on other animals stuck in the asphalt.”

More from Smithsonian.com:

Evolution World Tour: La Brea Tar Pits, California