hurwitz: the city bought up land around the reservoirs to prevent it from development. it provides assistance to local residents to see that there's no pollution of the reservoirs. it's much more cost effective to prevent pollution and to protect a source of water than to remove it at the drinking water treatment plant. lloyd: what epa said to us was, "you can have an exemption from filtration "if you keep this undeveloped, "and if you can manage the wastewater so that it does not pollute your water supply." and we feel that we've reached the point where we can really keep it clean enough to drink unfiltered for the indefinite future. and new york city is in a small club of cities that actually have that filtration avoidance waiver. narrator: while municipalities are responsible for maintaining systems and source supply, the standards that protect water are established at the federal level. there are two important pieces of federal legislation that were both enacted in the early to mid-1970s. the first was the clean water act, which acts to protect rivers and lakes, and

hurwitz: currently, city tunnel 1 and city tunnel number 2 would be feeding each half of the city. so you'd lose half the city if you didn't have a replacement. narrator: without half of its water supply, the city would shut down. for nearly 40 years, new york has been in the process of constructing a solution. man: this project is water tunnel number 3.

hurwitz: currently, city tunnel 1 and city tunnel number 2 would be feeding each half of the city. so you'd lose half the city if you didn't have a replacement. narrator: without half of its water supply, the city would shut down. for nearly 40 years, new york has been in the process of constructing a solution. man: this project is water tunnel number 3. we started on this project in 1969. i'm a sandhog. i've been a sandhog for 37 years. narrator: sandhogs are the men of local 147, who work deep below the city. they began building the infrastructure of new york in 1872. from the subways to the sewers, the water tunnels to the highway tunnels, new york city thrives because of their work. ryan: you got one little hole in the ground, and nobody knows we're here. see the empire state building, right. that's 1,000 feet. so you figure, you go down 1,000. how high that is -- that's how far we go down. narrator: stretching more than 60 miles under the city, tunnel 3 is taking generations of workers to complete. ryan: i don't even want to imagine what my father had to go through. when we fi

hurwitz: we have the stock exchange, we have the united nations -- failure can have a dramatic impact on the nation, and even internationally. so there's a really keen awareness that you always have to be fixing the system. things corrode, they rust. they get to where you turn them on and nothing happens. but it is so totally used in every nook and cranny, that making any accommodation to shut it down, to do something to it, is very difficult. narrator: two massive underground tunnels, called simply tunnel 1 and tunnel 2, provide most of the city's water supply. they run hundreds of feet below manhattan, far deeper than the subways. built at the beginning of the 20th century, they are concrete-lined and bored through solid rock. they could last centuries. but the mechanical equipment within them will not.

hurwitz: we have the stock exchange, we have the united nations -- failure can have a dramatic impactn the nation, and even internationally. so there's a really keen awareness that you always have to be fixing the system. things corrode, they rust. they get to where you turn them on and nothing happens. but it is so totally used in every nook and cranny, that making any accommodation to shut it down, to do something to it, is very difficult. narrator: two massive underground tunnels,

hurwitz: we have the stock exchange, we have the united nations -- failure can have a dramatic impact on the nation, and even internationally.