Both the Australian and Chinese navies have picked up beacon signals over the past three days, but time is running out and the challenge of location is immense. Retired Air Chief Marshal Angus Houston, head of the Australian agency coordinating the search for Malaysia Airlines Flight 370, acknowledged that the search area was essentially a best guess and noted that the time when the plane’s locator beacons would shut down was “getting pretty close,” the Associated Press reported.

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After an Australian vessel, Ocean Shield, again detected deep-sea signals consistent with those from an airplane’s black box, the official leading a multination search expressed hope Wednesday that crews will begin to find wreckage of a missing Malaysian airliner “within a matter of days.”“I believe we’re searching in the right area,” Retired Air Chief Marshal Angus Houston said. All commercial transport aircraft are fitted with underwater locator beacons to assist in the relocation of black box flight data recorders and cockpit voice recorders. These beacons are free-running pingers that transmit signals at an acoustic frequency of 37.5 kilohertz and have an expected battery life of 30 days. The scale of the challenge in locating the black boxes is immense. 1,000 feet 460 psi 2,000 feet 903 psi 3,000 feet 1,348 psi 4,000 feet 1,792 psi 5,000 feet 2,236 psi 1 mile down 6,000 feet 2,680 psi 7,000 feet 3,125 psi 8,000 feet 3,569 psi 9,000 feet 4,013 psi 10,000 feet 4,458 psi 2 miles down 11,000 feet 4,902 psi 12,000 feet 5,347 psi 13,000 feet 5,791 psi 14,000 feet 6,235 psi 6,680 psi 2,717 feet – the depth of an invertedBurj Khalifa — the world’s tallest building, located in Dubai, United Arab Emirates. 2,600 feet – the maximum known depth at which giant squids swim. 3,280 feet – the maximum known depth of a sperm whale dive.They are thought to be capable of remaining submerged for 90 minutes. 4,600 feet – the depth to which the towed pinger locator was lowered when the Ocean Shield’s crew was able to detect the signal for more than two hours Sunday, according to Cmdr. William Marks of U.S. 7th Fleet. 6,000 feet – is the depth that an underwater pinger locator would have to reach to hear the beacon on the bottom of the ocean, depending on environmental conditions, according to Hydro International magazine. 9,816 feet – the maximum known depth of the deepest diving mammal, the Cuvier’s beaked whale. 14,763 feet – the maximum dive depth of Alvin, the first deep-sea submersible capable of carrying passengers. 15,000 feet – just shy of three miles down. This is around the depth that the signal was detected, and the maximum known depth of the ocean floor below the Ocean Shield. 12,500 feet – the depth of the wreck of the Titanic. The Titanic sank after striking an iceberg on its maiden voyage to New York in April 1912.It took 73 years to locate the wreck. 13,100 feet – the depth at which the flight data recorders from Air France Flight 447 were found. The flight crashed in the Atlantic Ocean en route from Rio de Janeiro to Paris in 2009. The black boxes from the missing Airbus A330-203 took two years to locate. 555 feet – the depth of an inverted Washington Monument — it is the tallest structure in the District of Columbia. 22 feet – the draft of the Australian offshore support vessel Ocean Shield, now searching for the black box. It is 347 long. 200 feet – the widthof a Boeing 777-200. 1,250 feet – the depth of an inverted Empire State building — it was the tallest building in the world from 1931 to 1973. 1,600 feet – the test depth of the American Seawolf-class submarine

The search for Flight 370

The search for Flight 370 entered its fourth week Sunday with a growing group of planes and ships assisting in search.

Inside the investigation

One of aviation’s greatest mysteries began when Flight 370 took off into clear skies March 8 and seemed to disappear into thin air.

A black-box ping's distorted journey

A black box emits high-frequency signals that can create a complex pattern of sound waves under the ocean's surface.

China’s confused status in the investigation

Worldviews: China's "pings" not being investigated for now.

SOURCE: Australian Maritime Safety Authority, Hydro International magazine, National Oceanic and Atmospheric Administration Fisheries, BBC.co.uk, and Plosone.org. GRAPHIC: Richard Johnson and Ben Chartoff - The Washington Post.