While working on a housing development in Scotts Valley, a residential district high in the Santa Cruz Mountains of California, construction workers were amazed to discover the skeleton of a giant whale.

Archaeologists and paleontologists rushed to the scene to uncover a relatively intact skeleton, including skull, shoulder and arm bones, and the vertebrae.

Researchers have determined that it is a mysticete whale, the ancient forebear of the baleen whale. The whale is estimated to have been 25 feet long and to have lived 4 million years ago.

“That’s an interesting time in whale evolution,” said Matthew Clapham, a paleontologist at the University of California-Santa Cruz.. “A lot of whales were starting to evolve from their early ancestral group so this specimen, depending on how complete it is, could say a lot of interesting things about the evolution of whales.”

The whale is believed to have come to the mountains during one of California’s many tectonic plate shifts.

“Most places where you see a hill, somewhere there’s a fault line nearby pushing it up,” said Scott Armstrong, a scientist with Paleo Solutions. “They’re relatively inactive faults. But yeah, it’s from lifting thousands, maybe millions of years ago.”

The team still has some way to go before they can fully excavate the fossils. Using modern tools, they dug up the earth surrounding the bones and encased it with plaster. This allows the fossils to be safely transferred to an area where the paleontologists can finish excavating the fossils.

“If the bone is softer than the rock, it makes it very difficult because it’s hard to chip through the rock without breaking the softer bones, but we’ll get it,” said Armstrong.

“I think of the fossils you get along the coastline, it’s more common to get a piece of the skull or the brain case or some bones,” said Clapham. “So this sounds like a very impressive find.”

Baleen whales are currently the largest animals on Earth.