Imagine a massive island off the coast of California roughly thrice the size of Maui, a lush and wild place where miniature mammoths once roamed and ancient humans hunted in the shadows of towering trees. This island once existed and it’s called Santarosae, and while it is gone now, it was once a thriving ecosystem, teeming with life. Its story provides a captivating window into the ever-changing natural history of the California coast region.

During the last Ice Age, approximately 20,000 to 25,000 years ago, when sea levels were significantly lower, Santarosae Island was a single, expansive landmass that now comprises most of California’s Channel Islands. As the cooler Pleistocene climate transitioned into the warmer Holocene (the epoch we are in now), the Earth’s oceans heated and expanded. Continental ice sheets and glaciers melted, releasing vast amounts of water and causing sea levels to rise dramatically.

At its peak, Santarosae was massive—four of today’s Channel Islands (San Miguel, Santa Rosa, Santa Cruz, and Anacapa) were all connected into a single landmass. It spanned around 1,500 square miles, making it a significant feature of the Pacific coast landscape. Today, only remnants remain in the form of those four separate islands, but evidence of Santarosae’s ancient past continues to reveal itself to scientists.

Anacapa was the first to break away, around 10,300 to 10,900 years ago, as rising waters gradually submerged the narrow isthmus that once connected it to the rest of Santarosae. This slow disintegration of the super island was witnessed by the humans already inhabiting the region. Having arrived between 12,710 and 13,010 years ago, possibly even earlier, these early settlers likely traveled by boat, following the “kelp highway“—a rich, coastal ecosystem of underwater seaweed forests stretching from northern Japan and Kamchatka, along the southern shores of Beringia, down the Pacific Northwest, and into Baja California. For these early explorers, Santarosae would have appeared as a land of abundant resources.

One of the island’s most captivating features was its population of pygmy mammoths, found exclusively on Santarosae. Standing between 4.5 to 7 feet tall at the shoulder and weighing around 2,000 pounds, these miniaturized versions of mainland Columbian mammoths were about the size of a large horse and evolved to suit their isolated island habitat (see our story on the island biogeography of the Channel Islands). The reasons for their dwarfism stem from a phenomenon called island rule, where species on islands often shrink due to limited resources and isolation, as well as a shortage of predators. Despite their smaller size, these island-dwelling mammoths likely shared many characteristics with their larger relatives, including a similar body shape, short fur, and a large head. These mammoths roamed Santarosae until they disappeared around 13,000 years ago, coinciding with both climate changes and the arrival of humans.

The first discovery of “elephant” remains on Santa Rosa Island was reported in 1873. Over time, additional excavations provided insight into the island’s mammoth population, which gradually became smaller over generations, eventually disappearing at the end of the Pleistocene. Notably, paleontological digs conducted on Santa Rosa Island in 1927 and 1928 unearthed the remains of a new species, Mammuthus exilis. In the 1940s and 1950s, Philip Orr of the Santa Barbara Museum of Natural History recovered further specimens while conducting archaeological and geological work on the island.

Most pygmy mammoth remains have been discovered on Santa Rosa and San Miguel Islands, with fewer finds from Santa Cruz Island and even fewer from San Nicolas Island, which lies outside the Channel Islands National Park.

Santarosae was not just a wilderness for megafauna—it was home to some of the earliest known human settlers in North America. Archaeological discoveries, such as the remains of a 13,000-year-old woman unearthed on Santa Rosa Island, point to a sophisticated maritime culture. These ancient humans, likely ancestors of the Chumash people, navigated the waters around Santarosae in plank canoes, hunting seals, birds, and fish, while gathering plants and shellfish.

The island provided ample resources, but it wasn’t isolated from the rest of the world. The people of Santarosae were part of a complex trade network that stretched across the California coast. Evidence of these connections can be seen in the tools and materials found on the island, some of which came from distant sources. As sea levels rose, however, these early inhabitants had to adapt to the shrinking island, eventually migrating to the mainland.

Santarosae’s landscape during the Ice Age was strikingly different from what we see on today’s Channel Islands. Dense forests of pines, oaks, and other vegetation covered much of the island, supporting a rich diversity of life. The island’s topography included hills, valleys, and freshwater sources, offering an ideal environment for both humans and animals. As the climate warmed and sea levels rose, the island’s ecology shifted. Forests retreated, and the landscape began to resemble the wind-swept, scrubby terrain seen on the modern Channel Islands.

The rise in sea levels didn’t just transform the landscape; it also altered the ecosystems. Many of the animals, like the pygmy mammoths, couldn’t survive the changing conditions (or human hunters), while new species adapted to the shrinking landmass. Birds, insects, and plant species began to dominate, and the island ecosystems became more specialized.

Today, the remnants of Santarosae offer an invaluable window into the past. The Channel Islands National Park protects much of the area, and researchers continue to uncover clues about the island’s history. Ongoing archaeological digs and ecological studies on the islands help piece together the story of Santarosae’s people, animals, and landscape.

For those who visit the Channel Islands today, it’s hard to imagine the ancient world of Santarosae—a much larger island teeming with life. But the remnants of this lost island still hold secrets waiting to be uncovered, offering a fascinating glimpse into California’s distant past and a reminder of how the forces of nature continually reshape our world.

Though Santarosae is now submerged, its influence is still a significant part of California’s natural history.