Santa Barbara Channel, a rich area of marine life, lies west of the city of Ventura, California and south of Santa Barbara. As a part of the Pacific Ocean, it separates California's mainland from the northern Channel Islands. The Channel's east-west course is approximately 80 miles long and averages around 30 miles across. It becomes narrowest at the eastern extremity where Anacapa Island, formed by volcanic activity, is less than 15 miles off the coast of Ventura. The channel is a picturesque stretch of water; the islands are visible from the mainland on clear days. Whale-watching boats cross the channel daily from Ventura, while huge cargo ships and tankers travel the major shipping route on their way to or from the ports of Los Angeles and Long Beach. As early as the 15th century, the indigenous Chumash paddled "tomols" across the channel to Santa Cruz Island to fish. It was traveled by non-natives from well before the 16th century, and more than 100 shipwrecks have been discovered. A sampler of the channel's historical highlights includes a 30-foot-tall lighthouse constructed in 1856 to assist and protect coastal vessels; a wharf built in 1872 to off-load lumber and passengers from large schooners; the Santa Barbara Harbor, built in 1930; off-shore drilling attempts by oil industry leaders, following World War II. Santa Barbara Channel also was the site of a major oil spill in 1969. Union Oil’s Platform A oil well spilled 80 to 100 thousand barrels into the channel. Public indignation over the resultant massive environmental damage became a major spur to the incipient environmental movement.