City, making the then-small town attractive for recreational fishing. During World War II additional large defense facilities were established in the state such as Andrews Air Force Base, Patuxent River Naval Air Station, and the large Glenn L. Martin aircraft factory east of Baltimore. Mid-20th century In 1952, the eastern and western halves of Maryland were linked for the first time by the long Chesapeake Bay Bridge, which replaced a nearby ferry service. This bridge (and its later, parallel span) increased tourist traffic to Ocean City, which experienced a building boom. Soon after, the Baltimore Harbor Tunnel allowed long-distance interstate motorists to bypass downtown Baltimore, while the earlier Harry W. Nice Memorial Bridge allowed them to bypass Washington, D.C. Two beltways, I-695 and I-495, were built around Baltimore and Washington, while I-70, I-270, and later I-68 linked central Maryland with western Maryland, and I-97 linked Baltimore with Annapolis. Passenger and freight steamboat transportation, previously very important throughout the Chesapeake Bay and its many tributaries, came to an end in mid-century. Maryland, late 20th century to present In 1980, the opening of Harborplace and the Baltimore Aquarium made that city a significant tourist destination, while Charles Center, the World Trade Center, and the popular Camden Yards baseball stadium were constructed in the downtown area. Fells Point also became popular. The historic Annapolis waterfront area, previously a working-class fishing port, also became gentrified and a tourist destination. Baltimore's largest employer, the Bethlehem Steel factory at Sparrows Point, shrunk, and the General Motors plant closed, while Johns Hopkins University and Health Care System took Bethlehem's place as Baltimore's largest employer. There are over 350 biotechnology companies in the state. The Social Security – Health Care Financing Administration, Bureau of Standards, U.S. Census Bureau, National Institutes of Health, National Security Agency, and Public Health Service have their headquarters in the state. Metrorail lines were constructed in Montgomery and Prince George's counties, while Baltimore opened its own Metro Subway as well as the north–south Baltimore Light Rail system. In addition to general suburban growth, specially planned new communities sprung up, most notably Columbia, but also Montgomery Village, Belair at Bowie, St. Charles, Cross Keys, and Joppatowne, and numerous shopping malls, the state's three largest malls being Annapolis Mall, Arundel Mills and the Towson Town Center. Community colleges were established in nearly every county in Maryland. Large-scale, mechanized poultry farms became prevalent on the lower Eastern