Submerged California Coast Yields Thousands of Ice Age Stone Tools, Reshaping Early Human Arrival Timeline

Off the coast of La Jolla, divers working at roughly 80 feet down are finding stone tools on the ocean floor. The San Diego Archaeological Centre has reported more than 2,000 stone mortars and pestles, the bowls and grinding stones used to process food, lying on the seabed there. Taken together with other finds, researchers are describing thousands of submerged stone tools along California's coast.

The explanation lies in what happened after the last glacial maximum. According to California State Parks, melting ice caps pushed sea levels up by more than 300 feet. That was not simply more water in the ocean. It meant miles of land went under, including Indigenous settlement areas, and the coastline moved to where it sits today. Much of what once stood on that land is now buried beneath the sands of the continental shelf.

The tools at La Jolla are read as evidence that places now underwater were once inhabited sites, with villages and daily activity of Indigenous people over thousands of years. The mortars and pestles suggest long, repeated use of those places rather than a passing visit.

Farther north, the Northern Channel Islands tell a related story. During the Ice Age, glaciers linked them to the mainland, and they formed a single large island known as Santarosae. Scientists believe this landmass could have served as a kind of highway for the first people arriving in North America. Using sonar, researchers can now trace the original outlines of ancient rivers and forests on that vanished land, seeing where they lay before the ocean covered them.

The animals of that world are part of the record too. Megafauna such as Jefferson's ground sloth and mammoths lived on the land, and fossils of them can still be seen at the Santa Cruz Museum of Natural History. A well-known find from Santa Rosa Island includes mammoth tusks dated to about 100,000 years old. The source frames these animals as having been pushed to move by climate change and rising seas, much as people were.

The mapping work is being led in large part by federal expeditions from NOAA Ocean Exploration, which are trying to chart drowned valleys that might have held prehistoric camping places. According to NOAA's plans, scientists use side-scan sonar and sub-bottom profiling sonar. Both send sound waves through the water to the seafloor and produce maps of paleolandscapes, meaning the hills, valleys and riverbeds that existed before flooding. The source likens them to large-scale ultrasound machines for the ocean.

The larger claim is that this submerged record suggests early maritime travelers lived along these now-hidden coasts earlier than previously recorded, which would change the timeline of human arrival in North America. That is a significant assertion, and the source does not lay out the dating methods behind it. The 100,000-year-old figure applies to mammoth tusks, not to a confirmed human presence.

What to watch for is peer-reviewed publication of the dates. If they hold up, the drowned shelf off California could become central to the story of how people first reached the continent.