Boston is sometimes called a "city of neighborhoods" because of the profusion of diverse subsections; the city government's Office of Neighborhood Services has officially designated 23 neighborhoods. More than two-thirds of inner Boston's modern land area did not exist when the city was founded, but was created via the gradual filling in of the surrounding tidal areas over the centuries, notably with earth from leveling or lowering Boston's three original hills (the "Trimountain", after which Tremont Street is named) and with gravel brought by train from Needham to fill the Back Bay.
Downtown and its immediate surroundings consist largely of low-rise masonry buildings (often Federal style and Greek revival) interspersed with modern high-rises, notably in the Financial District, Government Center, and South Boston. Back Bay includes many prominent landmarks, such as the Boston Public Library, Christian Science Center, Copley Square, Newbury Street, and New England's two tallest buildings: the John Hancock Tower and the Prudential Center. Near the John Hancock Tower is the old John Hancock Building with its prominent illuminated beacon the color of which forecasts the weather. Smaller commercial areas are interspersed among areas of single-family homes and wooden/brick multi-family row houses. The South End Historic District is the largest surviving contiguous Victorian-era neighborhood in the US. The geography of downtown and South Boston was particularly affected by the Central Artery/Tunnel Project (known unofficially as the "Big Dig") which removed the unsightly elevated Central Artery and incorporated new green spaces and open areas.
Some of the most renowned and highly ranked universities in the world are located in the Boston area. Three universities with a major presence in the city, Harvard, MIT, and Tufts, are located just outside of Boston in the Cambridge/Somerville area known as the Brainpower Triangle. Harvard is the nation's oldest institute of higher education and is centered across the Charles River in Cambridge, though the majority of its land holdings and a substantial amount of its educational activities are in Boston. Its business, medical, dental, and public health schools are located in Boston's Allston and Longwood neighborhoods, and Harvard has plans for additional expansion into Allston.
The Massachusetts Instistute of Technology (MIT) originated in Boston and was long known as "Boston Tech"; it moved across the river to Cambridge in 1916. Tufts University's main campus is north of the city in Somerville and Medford, though it locates its medical and dental schools in Boston's Chinatown at Tufts Medical Center, a 451-bed academic medical institution that is home to both a full-service hospital for adults and the Floating Hospital for Children.