Two hours north of New York City, a mile-long stream and a marsh the size of a football field have mysteriously formed along a country road. They are such a marvel that people come from miles around to drink the crystal-clear water, believing it is bubbling up from a hidden natural spring.

The truth is far less romantic: The water is coming from a cracked 70-year-old tunnel hundreds of feet below ground, scientists say.

The tunnel is leaking up to 36 million gallons a day as it carries drinking water from a reservoir to the city. It is a powerful warning sign of a larger problem around the country: The infrastructure that delivers water to the nation’s cities is badly aging and in need of repairs.

The Environmental Protection Agency says utilities will need to invest more than $277 billion over the next two decades on repairs and improvements to drinking water systems. Water industry engineers put the figure drastically higher, at about $480 billion.

Water utilities, largely managed by city governments, have never faced improvements of this magnitude before. And customers will have to bear the majority of the cost through rate increases, according to an industry group, the American Water Works Association.

Engineers say this is a crucial era for the nation’s water systems, especially in older cities like New York, where some pipes and tunnels were built in the 1800s and are now nearing the end of their life expectancies.

“Our generation hasn’t experienced anything like this. We weren’t around when the infrastructure was being built,” a spokesman for the water industry group, Greg Kail, said. “We didn’t pay for the pipes to be put in the ground, but we sure benefited from the improvements to public health that came from it.”

He said the situation has not reached crisis stage, but without a serious investment, “it can become a crisis. Each year the problem is put on the back burner, the price tag is going to go up.”

Catastrophic problems can arise when infrastructure fails. An 84-year-old steam pipe erupted beneath a New York street last year, creating a mammoth geyser that rained mud and debris down on the city.

The 36 million gallons a day that leak from the 85-mile Delaware Aqueduct in New York State amounts to more than 1 billion gallons a month.

Residents in Wawarsing, about 100 miles from New York City, blame tunnel leaks for the constant flooding in their yards and basements. Department of Environmental Protection engineers are trying to determine whether the aqueduct is really responsible for the soggy mess along Route 209 that has gotten considerably worse over the last 10 years.

David Sickles said the water just bubbles up from the cracks in the concrete in his basement — even when it doesn’t rain.

“It’s like there is too much water in the ground already,” Mr. Sickles said, showing off the water line on the concrete wall of his basement. “There’s no place for this to go.”

Nearly every house has a black discharge hose running from the basement through the yard, gushing water into already-soggy patches of grass.

Utilites currently spend about $10.4 billion annually on large-scale repairs and improvements on drinking water infrastructure, a figure that has been relatively flat during the past two decades, the EPA said.

Cities have a hard time convincing residents that they should spend money on something they never see, buried hundreds of feet underground. And often, public officials pawn the responsibility off on the next person elected, Mr. Kail said.

Repairs tend to be long and costly, especially since many systems were built nearly a century ago, deep underground, where buildings and major roads now stand. Even monitoring pipes for vulnerabilities can be expensive and tricky, since it’s not possible to shut down a city’s water supply to test for leaks. If New York were to do that to the Delaware Aqueduct, for example, the 13 1/2-foot-diameter tunnel might crumble under the crushing weight of the land without the water to support the duct.

New York has spent decades digging a new $6 billion tunnel that will create an alternative source of water delivery and allow for easier inspection and repair of the other tunnels. It is expected to be completed by 2020.

Many engineers and water utilities say water bills around the country are too low. “We are the only utility where the raw material is free, but the infrastructure is the most expensive,” the chief executive water company Aqua America, Nick DeBenedictis, said. “We have to dig up streets in order to do it, but once we make investments it’s good for years.”