In the remote canyons east of Milpitas, scientists are digging up a treasure trove of marine fossils that tell of an era 20 million years ago when a shallow sea extended throughout much of today’s California.

More than 800 specimens have been recovered in the last three years from sandstone deposits on the site—including ancient whale bones, megalodon shark teeth, and scallops the size of dinner plates—as earth-movers recontour the land to prepare for the reconstruction of Calaveras Dam, a project of the San Francisco Public Utilities Commission. The work is part of a major seismic upgrade to the nearly century-old infrastructure supporting the city’s Hetch Hetchy water supply.

Among the noteworthy finds is a hippo-like creature, Desmostylus, that sported forward-facing tusks and weighed in at over 400 pounds. Paleontologist Mark Goodwin of the University of California Museum of Paleontology says the whale skeletons, both baleen and toothed, appear to be relatively small, though he’s not sure if the individuals were young when they died or if the taxa were small. The fossils have gone straight into protective casts until the SFPUC reaches agreement with a research institution to make it the repository.

“It was probably once a thriving seaway with a robust food chain supporting a number of marine animals,” says Lisa White, a Miocene expert at UCMP. The fossils will help piece together a more complete picture of what lived in and around that ancient ocean, she adds. That is, once they’ve reached their final resting place.