PHILADELPHIA — Francis McConnell is a field supervisor for the Philadelphia Water Department, but lately he is acting more like an undercover police officer.

Several hours a day, five days a week, he stakes out junkyards. Pretending to read a newspaper, Mr. McConnell sits near the entrances and writes down descriptions of passing pickup trucks and shirtless men pushing shopping carts.

Jessica Kourkounis for The New York Times

John Mock secures covers with chain on a Philadelphia street. More Photos »

His mission is to figure out who is stealing the city’s manhole covers and its storm drain and street grates, increasingly valuable commodities on the scrap market. More than 2,500 covers and grates have disappeared in the past year, up from an annual average of about 100.

Thieves have so thoroughly stripped some neighborhoods on the city’s north and southwest sides that some blocks look like slalom courses, dotted with orange cones to warn drivers and pedestrians of gaping holes, some nearly 30 feet deep.

Two adolescents were injured in recent months after falling into uncovered holes, motorists and cyclists are increasingly anxious about damaging tires, and the city is spending hundreds of thousands of dollars — $300,000 at last count — to replace the missing covers.

“They used to say the streets around here will swallow you up, but they were talking about drugs and guns,” said Keith Thomas, 32, as he hoisted a radiator he collected onto a scale at a junkyard in a drug-ravaged section of the Kensington neighborhood on the city’s north side.

City crews tried screwing down the covers with hexagonal bolts, but the thieves responded with Allen wrenches to unscrew them.

The city pressed scrap dealers to refuse material marked as city property; but the thieves adapted again, using blow torches to partially cut up or melt off the city labels.

One thing has helped. A Water Department worker, Fred Feoli, designed a way to lock the manhole covers from the inside. But so far, only 300 of the city’s more than 70,000 manhole and inlet covers have been locked.

So for now, Mr. McConnell is stuck with conducting the surveillance of junkyards.

“I’m here because the real police are too busy chasing serious crimes like shootings and murders,” said Mr. McConnell, craning his neck hoping to glimpse what was in the back of a van entering a scrap yard.

Thieves can get $5 or $10 for wrought-iron inlet covers, which weigh about 40 pounds and cover curbside drains. The larger manhole covers in the center of the streets weigh about double and triple that and are worth commensurately more.

The problem is playing out elsewhere too.

Phoenix has lost more than 160 of its manhole covers and street storm drains this year, up from 10 last year.

More than 80 drains and manhole covers have been stolen in Long Beach, Calif., this year and at least two local car owners who drove over the open chambers have filed claims against the city.

Starting last year, such thefts in Cleveland, Memphis, Miami and Milwaukee have more than doubled compared with other years, although New York reports no such increase.

“We have had our share of copper theft,” said Michael S. Clendenin, a spokesman for Con Edison in New York. But “New Yorkers are a pretty alert bunch and anyone trying to tuck a manhole under their arm in Times Square would look pretty suspicious.” He added that the utility’s covers weighed 300 pounds.

For most of the cities, the increase started in the spring of last year as the price of steel and iron surged because of a growing demand for recycled metals in China and India. Thieves have been pulling up anything metal — screen door frames, plumbing fixtures, copper wiring — they can get their hands on.

Long Beach is considering plastic covers, and Miami has started welding the covers in place. Cleveland is sealing manholes with tar, and Phoenix has assigned four police detectives to a task force that is investigating the thefts.

Several water districts have started offering cash rewards to whistleblowers who report attempted thefts, and the Institute of Scrap Recycling Industries, the industry’s largest trade organization, has begun sending alerts to scrap dealers whenever law enforcement officials inform the association of a theft.

State lawmakers are also taking action. At least 28 states have proposed bills this year increasing penalties for metal theft or requiring metal recyclers to fingerprint customers and keep better transaction records, according to the National Conference of State Legislatures. Sixteen of the bills have become law.

Conrad Stipp, the manager of Coach II Car, a junkyard in a gritty section of North Philadelphia, said junkyards were not to blame.

“The same people who are stealing from the city are stealing from me,” he said, pointing to a gaping hole in the ceiling of his office.

Two weeks ago, Mr. Stipp said, thieves cut a five-foot-wide hole through the roof of his building and stole all the copper he had, including the pipes in his office bathroom.

“You think I want to attract these types by accepting stolen city property?” he asked.

Across the street from Mr. Stipp’s junkyard, Mr. McConnell shook his head as he recounted how he confronted two men with a shopping cart full of manhole and inlet covers entering the yard two weeks ago.

When approached, the men said they had found the covers abandoned several blocks away already in the cart, Mr. McConnell said.

He said his department filed police reports every several weeks when city property was spotted being bought or sold. So far, he said, the city has made three arrests.

Listening to a description of surveillance efforts by the Water Department and recent arrests, John Sergeant, a scrap collector from North Philadelphia, laughed.

“These guys here,” Mr. Sergeant said, pointing at one scrap yard, “They’d buy a police cruiser and melt it down if we brought it in. The prices for metal are just that good these days.”

Stolen Statue Sold for Scrap

CHERRY HILL, N.J. (AP) — Thieves who stole a one-ton bronze statue of a horse from the former Garden State Park racetrack cut the statue into thousands of pieces, which they sold as scrap to a junkyard in Camden.

Sgt. Joseph W. Vitarelli, a Cherry Hill police detective, said the statue sold as scrap for a few thousand dollars, a small fraction of its value as art.

Three men were arrested Monday and charged in the theft. A fourth man had been arrested earlier.