Lyon Mountain’s ore is some of the purest in the world - it was used to build the Golden Gate Bridge. The mine itself was one of the deepest in North America, with its deepest shaft reaching 3100 feet. The ore was first mined from the surface in the 1860s, then quickly went underground. The population skyrocketed to nearly 3,500 people at its peak, as small settlements sprung up around the mining operation. During World War II, it was the largest employer in Clinton County.