The most important piece of infrastructure in New York City is a hole in the rock, and almost nobody alive has seen the inside of it.
It runs 85 miles from a reservoir in the Catskills to a holding basin in Yonkers, carries about 600 million gallons of drinking water a day, and does the entire job on gravity. No pumps. Gravity works because the Catskills sit higher than the city, and water only ever wants to go one way. That one tunnel, the Delaware Aqueduct, supplies roughly half of what nearly 10 million people drink, cook with and shower in, per the New York City Department of Environmental Protection.
Fiorello La Guardia opened the gates himself in 1944, and the water has barely stopped moving since.
How deep it actually goes
Start with the numbers, because they are faintly absurd. The Rondout-West Branch Tunnel, the 45-mile stretch doing the heaviest lifting, swings from as little as 300 feet below ground to as much as 2,300 feet in places, and slides beneath the Hudson itself at around 600 feet under the river’s surface, per the department’s own environmental review of the repair project.
It’s 13.5 feet across, concrete-lined, and pressurised by the elevation drop from one end to the other, still running after eight decades of near-continuous service. All of which rules out the obvious inspection method: sending a person down with a torch and a clipboard.
The torpedo that took 160,000 photographs
So the city sent a robot instead.
In June 2003, engineers lowered a self-steering submarine into a shaft near Rondout Reservoir at five in the afternoon and hauled it out 16 hours later near West Branch Reservoir in Putnam County, 45 miles downstream. Someone named it ULIISYS, for Underwater Linear Infrastructure Investigation System, an acronym tortured enough to deserve a medal. On the way through, it took more than 160,000 digital images and logged pressure, velocity and sound.
The vehicle carried no tether. It was built on the REMUS platform developed at the Woods Hole Oceanographic Institution, the same family of machines the US Navy used to hunt mines in the harbour at Umm Qasr.
A rebuilt version ran the route again in 2009 with upgraded computers, sensors and camera strobes, taking a frame every eight feet and finishing with roughly 180,000 photographs. A remotely operated vehicle caught it at the far end and carried it up.
Cracks, leaks and a price tag
So what did all those photographs actually show? A 2007 audit by the New York State Comptroller’s Office recorded that the 2003 survey had turned up a heavily cracked stretch running about 7,000 linear feet, most of the damage clustered in two zones sitting next to geological faults.
Two leaks have been monitored since 1992, one beneath the hamlet of Roseton in the Town of Newburgh and a smaller one at Wawarsing in Ulster County. Between them they can shed upwards of 35 million gallons a day, nearly all of it disappearing into bedrock beside the Hudson. Catskill rainfall travels 60-odd miles to be lost about 700 feet underground.
The same audit put a price on that. At the rate the city charged for metered water, the leakage at full flow worked out at somewhere between $24 million and $28 million a year. DEP officials argued that no dollar figure should be attached at all, on the grounds that supply comfortably exceeded demand.
The $14 million robot nobody used
Then there is the Chiton. The AUVs flown in 2003 and 2009 followed a fixed route and left engineers to study the photographs after the vehicle was already out of the tunnel, with nobody watching in real time. What the DEP wanted next was something tethered and steerable that could stream live video as it moved, and that’s what it commissioned.
Writing in Delaware Currents, Chris Mele detailed the remotely operated vehicle that resulted: 580 pounds, built from jointed segments, named after a segmented mollusc, designed to squeeze through a narrow shaft restriction and travel three miles to the cracked ground.
Michael Lombardi, an undersea specialist who worked on the later stages of the project, told Mele the insertion alone required “precision akin to landing on the moon”. He also said the department took delivery in 2018 and never put the thing in the water.
DEP’s reply, through spokesman John Milgrim, was that the Chiton remained complicated and untested, that its manufacturer failed to deliver on most of its contractual commitments, and that the hardware sits in warehouse inventory. Lombardi contests that version in a comment beneath the article. What exists on the public record is a disagreement between one consultant and one agency, which isn’t the same thing as a settled account.
Waiting on rain
By 2021, the actual fix already existed: a 2.5-mile bypass tunnel bored beneath the Hudson, running alongside the original. It just wasn’t attached to anything.
Making the connection means shutting the aqueduct down for eight months and draining it, starting in October, when demand for water bottoms out. October 2024 was the date. Work began. It stalled in November when a historic autumn drought hit, and the construction contract expired along with it. The department went back to procurement in May 2025 and now says the connection won’t be finished until after 2027.
Half the city’s water can only go offline in a year when the other half’s reservoirs are full enough to cover for it. So the largest repair job in the history of New York’s water supply now waits on the weather, while 80-year-old concrete sits in the dark with a camera drone for company.
About this article
This article is for general information and reflection. It is not professional advice. For your specific situation, consult a qualified professional.
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