Sulphur Springs Mountain in southwestern Solano County rises above Vallejo, offering hiking access from Blue Rock Springs Park and wide views over the northern San Francisco Bay and Napa River. Its basaltic lava and serpentine outcrops make it geologically important, and serpentine soils support rare and endangered species. The mountain also preserves the remains of three Native American village sites linked to the Suisunes, the Coast Miwok, and other Patwin peoples. In the early 1900s, cinnabar mining at Hastings Mine and St. John’s Mine left extensive underground shafts, while heavy use of the sulfur spring and later inspections revealed lasting environmental impacts, including mercury contamination.