The San Francisco Bay is a shallow, fault bounded, productive estuary. Structurally, it is controlled by the San Andreas transform fault to the west and the Hayward fault to the east. It drains water from approximately 40 percent of California, flowing down the Sacramento and San Joaquin rivers from the Sierra Nevada mountains, to enter the Pacific Ocean. The entire group of interconnected bays are often referred to as the San Francisco Bay. Located in California, San Francisco Bay is surrounded by a region known as the San Francisco Bay Area. The bay covers somewhere between 400 and 1,600 square miles, depending on which sub-bays, estuaries, wetlands, and so on are included in the measurement. The main part of the Bay measures three to 12 miles wide east to west and somewhere between 48 miles and 60 miles north to south. The wetlands and inlets of the bay have been gradually, deliberately, and systematically filled in, changing the bay's size since the mid-1800s by as much as one third or more. Large areas of wetlands have been restored, which further confuses the issue of the bay's size. The thousands of acres of marshy wetlands forming the edges of the bay were considered for many years to be wasted space. As a result, soil excavated for building projects or dredged from channels was often dumped onto the wetlands and into other parts of the bay as landfill. From the mid-1800s through the late 1900s, more than a third of the original bay was filled and often built upon. The deep, damp soil in those areas is subject to liquefaction during earthquakes, and most of the major damage close to the bay in the Loma Prieta earthquake of 1989 occurred to structures located there. The first recorded European discovery of the San Francisco Bay was on November 4, 1769, when Spanish explorer Gaspar de Portolà, unable to find the port of Monterey, California, moored his ship close to what is now Pacifica. Portolà and an expeditionary crew of 63 men and 200 horses began an overland journey that took them to the summit of the 1,200-foot high Sweeney Ridge, where he sighted the San Francisco Bay. Sweeney Ridge is located in northern San Mateo County, and is now a part of the Golden Gate National Recreation Area where a monument marks the discovery site. The site is listed on the National Register of Historic Places. It is believed that the Spanish explorer Juan de Ayala was the first European to enter the bay. He passed through the Golden Gate on August 5, 1775, in his ship the San Carlos, and moored in a bay of Angel Island, now known as Ayala Cove. After numerous unproductive efforts to purchase the bay and varying areas around it, the U.S. Navy and Army seized the region from Mexico during the Mexican-American War (1845-1848). On February 2, 1848, California seceded from Mexico. A year and a half after gaining independence, California requested to join the United States on December 3, 1849, and was accepted as the 31st state of the Union on September 9, 1850. The bay's regional importance became paramount when in 1869 the transcontinental railroad located its western terminus in Oakland. The San Francisco Bay Area is the American West's second-largest urban area with approximately eight million residents. San Francisco Bay and the Sacramento-San Joaquin Delta remain perhaps California's most important ecological habitats. California's Dungeness crab, Pacific halibut, and Pacific salmon fisheries rely on the bay as a nursery. The few remaining salt marshes now represent most of California's remaining salt marsh, supporting a number of endangered species and providing such key ecosystem services as filtering pollutants and sediments from the rivers. The bay is a key link in the Pacific Flyway. Millions of waterfowl annually use the bay shallows as a refuge. San Francisco Bay provided the nation's first wildlife refuge, Oakland's artificial Lake Merritt and America's first urban National Wildlife Refuge, the San Francisco Bay National Wildlife Refuge. San Francisco Bay is spanned by five bridges: the Golden Gate, Richmond-San Rafael, San Francisco-Oakland Bay, Hayward-San Mateo, and Dumbarton bridges. Four large islands exist in San Francisco Bay: Alcatraz, Yerba Buena, Treasure Island, and Angel Island. The San Francisco Bay is a mecca for sailors. Yachting and yacht racing are popular pastimes.