At the original site of mining activity in New Almaden, Ohlone Indians extracted cinnabar ore from a "red cave." In 1845, while visiting Mission Santa Clara, Mexican soldier Andres Castillero observed Ohlones using crushed cinnabar as a paint pigment, recognized its significance, followed them to this site, claimed his "discovery," and began mining the cinnabar ore, which contained mercury, a valuable liquid metal needed by Mexico to process silver and gold. The Main Tunnel became the principal entrance to the New Almaden Mine, which grew to become the richest mining area in all of California.