Authorities are all but certain Malaysia Airlines Flight 370 went down in the south Indian Ocean in water that may be as deep as 23,000 feet. That makes finding the all-important "black box" flight data recorder infinitely more difficult, and a job perfectly suited to the U.S. Navy's tow fish.

The 70-pound tow fish, which is formally known in true Pentagon style as Towed Pinger Locator 25, is a hydrodynamic microphone designed specifically to listen for the acoustic signal of the data and cockpit voice recorders carried aboard all commercial and military aircraft. It can track the devices to depths of 20,000 feet.

"Basically, this super-sensitive hydrophone gets towed behind a commercial vessel very slowly and listens for black box pings," says Commander Chris Budde, U.S. 7th Fleet operations officer.

The U.S. Navy deployed a pair of tow fish aboard the Seahorse Standard, a Royal Australian Navy Rescue Support vessel that will drag it through the search area west of Perth, Australia. The Standard joins a flotilla of a dozen ships scouring a vast swath of sea for any sign of the Boeing 777-200ER, which vanished March 8 enroute to Beijing from Kuala Lampur.

Although satellite images have revealed debris floating in the area, authorities have so far found no sign of the airplane or the 239 people aboard.

The Seahorse Standard will drag a TPL 25 through the search area at around 3 knots, while a second is on-hand as a backup. The device, tethered to the ship by 20,000 feet of cable, remains about 1,000 feet above the sea floor, listening for the telltale ping of the underwater locator beacon installed on black boxes (they're actually orange) and cockpit voice recorders. It can detect a transponder signal between 3.5 and 50 kHz (most commercial airliner data systems transmit at 37.5 kHz) within a 2-mile radius, and cover about 150 square miles of ocean each day.

"We have worked off of that ship before," says Mike Dean, the U.S. Navy's Supervisor of Salvage and Diving. "It's a good platform and a good crew."

TPL 25 is the third iteration of the technology that's been around for 20 years.

"We've used it for just about any aircraft that's gone down," Dean says.

It was instrumental in finding TWA flight 800 off the coast of New York after it crashed in 1996, although at a relatively shallow depth of 130 feet. It also was deployed during the search for Air France Flight 447 which crashed into the Atlantic Ocean in 2009. It was less successful there, however, because the underwater locator beacon, which is activated the moment it's submerged, has enough power to transmit for 30 days or so. The search for the missing Airbus A330 took two years.

Beyond the limitations of the beacon, there's also the fact no one knows just where Flight 370 went down, or where the strong currents of the south Indian Ocean might have carried the debris. The search area covers some 35,400 square miles–roughly the size of Massachusetts and West Virginia combined.

"There's an awful lot of ocean to cover," Dean tells WIRED.

, will use the TPL Photo: Royal Australian Navy

Homepage image: original image: U.S. Navy; illustration: WIRED