Their main job used to be retrieving rusty murder weapons, the corpses of homicide victims and the occasional stolen car from the murky depths of the city's waterways.
But in recent years, the New York Police Department's scuba team has quietly adopted a new role: scouring seawalls, bridge footings and ship hulls every day for explosives.
The team's mission has changed dramatically since the Sept. 11, 2001, terrorist attacks as the NYPD launched a series of ambitious counterterrorism efforts. The 37,000-officer department has reassigned 1,000 officers to counterterrorism, and also retooled aviation and harbor security, and given emergency units more weaponry, to help detect and deter threats.
"We're configured to respond in significant numbers, literally, by land, sea and air," said Police Commissioner Raymond Kelly.
The NYPD recently demonstrated that shift by buying four underwater robots, or ROVs (Remotely Operated Vehicles), fitted with cameras to help the 31-diver scuba team search for explosives. Four more of the underwater robots, which cost $75,000 each, have been ordered.
The ROVs look like mini submarines with dual propellers, wide-angle lenses and halogen lamps. Once submerged to depths reaching 200 feet, they beam up live color video through an umbilical cord to a 17-inch screen. Separate sonar images appear on a laptop computer.
Officers steer the device with a joystick - a feature friendly to "guys who are good at video games," said Bobby Harris, one of several detectives who double as divers.
On a recent morning in lower Manhattan, Harris was at the helm of a 55-foot NYPD boat sent to inspect a busy heliport about a hulking barge. The president's helicopter, Marine One, sometimes lands there.
After two divers in dry suits checked the perimeter of the barge and an adjoining pier, an underwater camera robot was dropped into the water to take a closer look at the pier pilings in deeper water. The images were clear, but what the police saw was innocuous: mostly mussels and algae.
The team has also used the camera to inspect the base of the Brooklyn Bridge, ferry landings outside Yankee Stadium on game days, the cruise terminal where the Queen Mary 2 has docked, a retaining wall on the East River beneath the United Nations, and several other sites.
The divers are part of the Harbor Unit, which patrols 476 square miles of water and more that 500 miles of waterfront. The area includes the East and Hudson rivers and New York Harbor, home to the Statue of Liberty and Ellis Island.
The team makes about 1,000 dives a year, and it's no Caribbean: The water can be chilly, choppy and so murky "you can't see your hand in front of your face," said Detective Robert Rodriguez.
Divers tell war stories about the TWA Flight 800 disaster, in which a Paris-bound Boeing 747 exploded off Long Island in 1996; the heartbreaking search for four doomed teens whose rowboat sank in a Bronx bay in 2003; and a search in a mob investigation about 10 years ago that turned up a watery graveyard of 20 sunken cars, including one with a murder victim in the trunk.
The team boasts that it is one of the few in the country to dive out of helicopters in 80 pounds of gear during rescue missions. The gear includes knives designed to break windows and cut through seat belts to free passengers of sunken vehicles.
In their new role, the divers have been trained to recognize limpet mines, or small bombs that a swimmer could attach to ship hulls and other surfaces. Though they say there have been no specific threats made or explosives recovered, police officials believe such explosives are in the terrorist arsenal.
The team also has been working with NYPD intelligence officers to identify and sweep large merchant ships that might be smuggling narcotics or more dangerous cargo.
"If they can bring in drugs, they can bring in other materials," said Lt. John Hawkins, commander of the scuba team.