The Calaveras Reservoir east of Milpitas, California, is set to become home to a new dam project. But workers digging up ground for the structure have discovered something unexpected - a treasure trove of ancient fossils.

Fossils discovered at the construction site are providing researchers a glimpse into what life was like in the area now called Silicon Valley, 20 million years in the past.

A total of 529 species of fossils have already been found at the site. Vertebrae species account for 168 of these, and 267 were invertebrates. One of the species discovered there are called Desmostylus. These ancient mammals were herbivores, much like the modern hippopotamus. This gentle giant lived in the area between 28.4 and 7.25 million years ago.

Shark teeth from 40 million years in the past have also been found there, along with fossilized barnacles and clams. Scallop-like fossils have been uncovered, some of which are 12 inches in diameter. One find could even consist of the entire fossilized skeleton of an ancient whale.

"We started finding fossils here before construction even started. It was exciting. We were finding scallops, and I said, 'I want to get a whale.' And we did," Jim Walker, a paleontologist leading the scientific expedition, said.

A total of nine whale skulls have been found at the site so far. Finds also include 39 species of plants and 55 other artifacts of the past, including footprints and preserved animal dens, of various species.

Work started on the new dam at Calveras Reservoir in 2011. The structure, when complete, will stand 220 feet tall, roughly the same size as the dam it is replacing. Each of the structures is made from compressed dirt and rock. The project is being managed by the San Francisco Public Utilities Commission.

The area is closed to the public, due to construction hazards. Paleontologists, geologists and other researchers are accompanying workers as they carve out ground, in order to minimize damage to the artifacts. They will continue to guide construction for the next two to three years, as the project continues. The new structure is due to be completed in 2018, replacing the old Calveras Dam, built in 1925.

The dams are part of the Hetchy water system, which supplies fresh water to 2.6 million people. Workers dug a hole in front of the old dam, 500 feet deep. The five million cubic yards of dirt they moved could have filled half a million dump trucks. Operations triggered a pair of landslides, each of which revealed vast quantities of new fossils.