San Francisco Bay is a shallow, productive estuary through which water draining from approximately forty percent of California, flowing in the Sacramento and San Joaquin rivers from the Sierra Nevada mountains, enters the Pacific Ocean. Technically, both rivers flow into Suisun Bay, which flows through the Carquinez Strait to meet with the Napa River at the entrance to San Pablo Bay, which connects at its south end to San Francisco Bay, although the entire group of interconnected bays are often referred to as "San Francisco Bay." San Francisco Bay is located in the U.S. state of California, surrounded by a contiguous region known as the San Francisco Bay Area, dominated by the large cities San Francisco, Oakland and San Jose. The Bay covers somewhere between 400[1] and 1,600[2] square miles (1,040 to 4,160 square kilometers), depending on which sub-bays (such as San Pablo Bay), estuaries, wetlands, and so on are included in the measurement. The main part of the Bay measures 3 to 12 miles (5 to 20 km) wide east-to-west and somewhere between 48 miles (77 km)1 and 60 miles (97 km)2 north-to-south. The bay was navigable as far south as San Jose until the 1850s, when hydraulic mining released massive amounts of sediment from the rivers that settled in those parts of the bay that had little or no current. Later, wetlands and inlets were deliberately filled in, reducing the Bay's size since the mid-1800s by as much as one third. Recently, large areas of wetlands have been restored, further confusing the issue of the Bay's size. Despite its value as a waterway and harbor, many 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 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 on. The deep, damp soil in these 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 on these areas. The Marina District of San Francisco, hard hit by the 1989 earthquake, was built on fill that had been placed there for the Panama-Pacific International Exposition (1915), although liquefaction did not occur on a large scale. In the 1990s, San Francisco International Airport proposed filling in hundreds more acres to extend its overcrowded international runways in exchange for purchasing other parts of the bay and converting them back to wetlands. The idea was, and remains, controversial. (For further details, see the "Bay Fill and Depth Profile" section.) There are four large islands in San Francisco Bay. Isolated in the center of the Bay is Alcatraz, the site of the famous federal penitentiary. Mountainous Yerba Buena Island is pierced by a tunnel linking the east and west spans of the San Francisco – Oakland Bay Bridge. Attached to the north is the artificial and flat Treasure Island, site of the 1939 Golden Gate International Exposition. Closest to shore, Angel Island was known as "Ellis Island West" because it served as the entry point for immigrants from East Asia. Raccoon Strait, between Tiburon and Angel Island, is the deepest part of the Bay. The federal prison on Alcatraz Island no longer functions, and the complex is now a popular tourist site.