These incredible vintage photos show how one of the world’s greatest engineering feats was created in 1906-1915 to bring water to New York City. In 1905, the city’s newly established Board of Water Supply launched the Catskill Aqueduct project, which would play an additional role in supplying the city’s ever-growing population of residents and visitors.

These projects rank as the greatest municipal water-supply enterprise ever undertaken, and as an engineering work is probably second only to the Panama Canal.

In 1898, Greater New York with a population of 3.5 million was formed through the consolidation of Manhattan, the Bronx, Queens, Brooklyn, and Staten Island. New York water engineers in the early decades of the twentieth century began to look further afield, to sources in the Catskill Mountains beyond the Hudson River.

The area in question was formerly a farming area, with logging activity as well as the quarrying of bluestone. Over two thousand people were relocated, including a thousand New Yorkers with second homes. 32 cemeteries were unearthed and the 1,800 residents buried elsewhere, to limit water contamination. Residents were offered $15 from the city disinter their relatives and rebury them elsewhere.

Buildings and industries were relocated or burned down, trees and brush were removed from the future reservoir floor–all the work done predominantly by local laborers, African-Americans from the south and Italian immigrants.

Officially the construction of the new water supply commenced in 1907. The aqueduct proper was completed in 1916 and the entire Catskill Aqueduct system including three dams and 67 shafts was completed in 1924. The total cost of the aqueduct system was $177 million ($2.4 billion in 2015 dollars).

The 92-mile (148 km) aqueduct consists of 55 miles (89 km) of cut and cover aqueduct, over 14 miles (23 km) of grade tunnel, 17 miles (27 km) of pressure tunnel, and nine miles (10 km) of steel siphon.

The 67 shafts sunk for various purposes on the aqueduct and City Tunnel vary in depth from 174 to 1,187 feet (362 m). Water flows by gravity through the aqueduct at a rate of about 4 feet per second (1.2 m/s).

In 1914, the wings of the dam were terraced with three- to four-ton blocks of bluestone quarried locally, and 780 men and 244 mules and horses laid macadam over the 40 miles of reservoir roads. The local water was so pure that New Yorkers had long been buying it in five-gallon carboys from the Crystal Spring Water Company of Pine Hill.

So the facility did not need a filtration plant, but was instead given an aerating fountain. Water passed through an aeration basin, a small reservoir 500 by 250 feet, its bottom lined with pipes four or five feet apart, from which streams were ejected 40 to 60 feet in the air.

This oxidized vegetable organisms and removed taste and odor. (Eventually alum was introduced to counter the effects of turbidity and soda ash to prevent over-acidity, and the water was chlorinated twice, at the Kensico and Hillview Reservoirs.)

On June 24, 1914, all the steam whistles in the zone were set off at once, marking the official end of the project. Only the cleanup remained. Throughout 1916, workers slowly demolished the camp at Brown’s Station and the plants and temporary railroads.

The year 1914 had been dry, but in 1915 the rains came, filling the reservoir to a hundred feet; on November 22, water was released into the aqueduct. The project was rivaled only by the Panama Canal as an achievement of America’s engineering might.

The Ashokan Reservoir had the potential to deliver 660 million gallons a day. The reservoir’s surface was equal to that of Manhattan below 110th Street. Its contents could fill the Hudson River from the Battery at the southern tip of Manhattan to Hastings-on-Hudson in Westchester County.

It took three days for a theoretical drop to travel from the Catskills to Staten Island, which received its first Ashokan water in January 1917.

In October 1917, New York City held a three-day celebration of its new water supply, during which 15,000 schoolchildren and a thousand women from Hunter College participated in a pageant in Central Park called “The Good Gift of Water.” As additions to the original, Schoharie Reservoir and Shandaken Tunnel was put into use 13 years later in 1928.

Nowadays, the Catskill Aqueduct has an operational capacity of about 550 million US gallons (2,100,000 m3) per day north of the Kensico Reservoir in Valhalla, New York. Capacity in the section of the aqueduct south of Kensico Reservoir to the Hillview Reservoir in Yonkers, New York is 880 million US gallons (3,300,000 m3) per day.

The aqueduct normally operates well below capacity with daily averages around 350–400 million US gallons (1,500,000 m3) of water per day. About 40% of New York City’s water supply flows through the Catskill Aqueduct.