Manhattan's water may have the same great taste New Yorkers have come to expect when they turn on their tap but now it's delivered through a 21st century system.

The city's Department of Environmental Protection yesterday completed the second phase of its much delayed project to create a new water tunnel that will alleviate the flow of its aging infrastructure.

At 5 p.m., the mayor activated the second part of Water Tunnel No. 3, which until Wednesday only served northern Manhattan, Astoria and some of the Bronx by delivering water from a Yonkers reservoir down to the city.

Bloomberg said the upgrade is significant because now all of the island will no longer be dependent on Water Tunnel No. 1, created in 1917 and now needs maintenance. "Millions of New Yorkers and visitors from the Battery all of the way to up Central Park will have a reliable supply of water," the mayor said.

Work on the tunnel, which will eventually stretch 32 miles in four boroughs, began in the 1970s, but funding dried up and delayed the completion.

The project -- one of the city's largest infrastructure projects and has cost $4.7 billion so far -- finished its first stage in 1998. Work on the now-completed phase two, which covers the rest of Manhattan, began in 2003.

Bloomberg said he made it a priority to speed up the construction because of the age of Water Tunnel No. 1, which is only one of two clean water tunnels in the city.

Although the nearly century old system doesn't have any major problems, the mayor said the city shouldn't depend on it 100%.

"If we were to lose one of the tunnels without backup, that part of the city would be uninhabitable," he said.

Work is already underway for the third phase of Tunnel No. 3, which would connect Brooklyn and Queens to the system.

Officials said construction would be complete by 2021 at the earliest.

DEP Commissioner Carter Strickland said crews will temporarily shut down Tunnel No. 1 and to make sure it is safe and up to date.

"Water is the lifeblood of New York City. None of the businesses, landmarks or attractions that make New York City the best city in the world . . . would be possible without clean water," he said.

The tunnel and other DEP projects were funded by rises in the water rates and the mayor acknowledged that New Yorkers had to pay a high price to maintain a high quality of water.

He assured them that the investment has paid off. "We should be able to sleep easier because of this," he said.

You may not notice the nearly 40 years of construction going on 500 feet below the streets of Manhattan but it is actually one of the biggest and most important infrastructure projects in the Big Apple. Here are some facts about the multibillion-dollar Water Tunnel No. 3:

It is made as a complementary tunnel to Water Tunnel No. 1, which is nearly a century old and due for maintenance. The tunnel has provided water to all the boroughs except Staten Island.

Plans for Water tunnel 3, which will be 32 miles long and directly provide water to Queens, the Bronx and Brooklyn as well as Manhattan, were first proposed in 1954 but work didn't begin until 1970.

The tunnel includes tools that allow for maintenance of specific parts of the tunnel. Phase one, which runs 13 miles and covers the Bronx, northern Manhattan and Astoria, was completed in 1998.

Phase two began construction in 2003 and used a boring machine to excavate a 12-foot diameter tunnel. Crews excavated more than 82 million cubic feet of soil and rock.

Construction on the third phase will be complete by 2021 at the earliest.

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