Millions of years ago, vents and fissures opened under the Pacific Ocean and lava flowed forth, creating huge underwater volcanic mountains and ranges called seamounts. The Farallon tectonic plate that formed a part of the Pacific Ocean floor (separate from the Pacific plate) inched eastward toward North America about 35 million years ago and most of the sea floor subducted beneath the continental land mass of the North America plate. Some of the sea floor, however, was scraped off and jammed against the mainland, creating the dome that was the forerunner of today’s Olympics.