See how one of the world's greatest engineering feats was created in 1915 to bring water to New York City

Two hours by car from Manhattan, 40% of New York City's water supply sits in the pristine Ashokan Reservoir.

From there, it travels through over 100 miles of tunnels to reach the city's 8.4 million residents.

Remarkably, the Catskill Aqueduct, which brings water from the Ashokan and Schoharie reservoirs, has been in operation since 1915.

The New York Public Library released almost 200,000 digitized images this week, and among them were these incredible photos of the Catskill Aqueduct construction, which began in 1906.

The NYC aqueduct system uses the same basic principle that the Romans used to bring water to their capital city: gravity. Only 5% of New York’s water needs to be pumped; the rest simply flows downhill.

But New York engineers had one thing the Romans didn't: dynamite. It allowed construction workers to bury the tunnels underground instead of needing to construct elevated aqueducts that snaked around mountains.

The Catskill Aqueduct tunnels are 30 feet across at their widest points — enough to make men standing inside them look tiny.