The Salinas River running alongside the central coastline of California is the largest underground river in North America, stretching for about 175 miles.

The Salinas River, which flows along the central coastline of California, is widely recognized as the largest "underground" river in North America. While it appears as a typical river on the surface during periods of heavy rain, for much of its 175-mile length, the water actually flows through a deep layer of sand and gravel beneath the visible riverbed. This unique geological characteristic has earned it the nickname "The Upside-Down River." The subsurface flow is a critical component of the Salinas Valley's hydrology, replenishing the vast Salinas Aquifer that supports one of the most productive agricultural regions in the United States. In recent years, due to prolonged drought conditions in 2025 and 2026, the visible surface water has become even rarer, making the underground "hidden" portion of the river the primary source of life for the surrounding "Salad Bowl of the World."