Back Bay is an officially recognized neighborhood of Boston, Massachusetts famous for its rows of Victorian brownstone homes — considered one of the best-preserved examples of 19th-century urban design in the United States — as well as numerous architecturally significant individual buildings and important cultural institutions such as the Boston Public Library. It is also a fashionable shopping destination, and home to some of Boston’s tallest office buildings, the Hynes Convention Center, and numerous major hotels.
Prior to a monumental 19th-century filling project, what is now the Back Bay was a literal bay. Today, along with neighboring Beacon Hill, it is one of Boston’s two most expensive residential neighborhoods.
The Neighborhood Association of the Back Bay considers the neighborhood’s bounds to be “Charles River on the North; Arlington Street to Park Square on the East; Columbus Avenue to the New York New Haven and Hartford right-of-way (South of Stuart Street and Copley Place), Huntington Avenue, Dalton Street, and the Massachusetts Turnpike on the South; Charlesgate East on the West.”
Before its transformation into buildable land by a 19th-century filling project, the Back Bay was literally a bay, located west of the Shawmut Peninsula (on the far side from Boston Harbor) between Boston and Cambridge, with the Charles River entering from the west. This bay was tidal: the water rose and fell several feet over the course of each day, and at low tide much of the bay’s bed was exposed as a marshy flat. As early as 5,200 years before present, Native Americans built fish weirs here, evidence of which was discovered during subway construction in 1913 (see Ancient Fishweir Project and Boylston Street Fishweir).
In 1814, the Boston and Roxbury Mill Corporation was chartered to construct a milldam, which would also serve as a toll road connecting Boston to Watertown, bypassing Boston Neck. However, the project was an economic failure, and in 1857 a massive project was begun to “make land” by filling the area enclosed by the dam.
The firm of Goss and Munson built 6 miles (9.7 km) of railroad from quarries in Needham, Massachusetts; 35-car trains arrived in the Back Bay 16 times each day, carrying gravel and other fill. (William Dean Howells recalled “the beginnings of Commonwealth Avenue, and the other streets of the Back Bay, laid out with their basements left hollowed in the made land, which the gravel trains were yet making out of the westward hills.”)  Present-day Back Bay itself was filled by 1882; the project reached existing land at what is now Kenmore Square in 1890, and finished in the Fens[vague] in 1900. Much of the old mill dam remains buried under present-day Beacon Street. The project was the largest of a number of land reclamation projects which, beginning in 1820, more than doubled the size of the original Shawmut Peninsula.
Completion, in 1910, of the Charles River Dam converted the former Charles estuary into a freshwater basin; the Charles River Esplanade was constructed to capitalize on the river’s newly enhanced recreational value.[vague] The Esplanade has since undergone several changes, including the construction of Storrow Drive.