In this handout image provided by the U.S. Navy, A Phoenix International worker lowers the U.S. Navy Supervisor of Salvage and Diving towed Pinger Locator into the ocean during operational testing April 1, 2014 in the Indian Ocean.
Crews searching for the missing Malaysia Airlines jetliner have just a few days left before pings from the plane's two data recorders could stop transmitting.
Today two ships with sophisticated equipment began searching underwater in a remote stretch of the Indian Ocean.
Former Australian Air Chief Marshal Angus Houston says an underwater search is underway
"The Royal Australian Navy and Royal Navy have today commenced a sub-surface search for emissions from the Black Box pinger from Malaysia Airlines Flight MH 370," Houston said.
Malaysian Flight 370 disappeared nearly four weeks ago with 229 people on board.
The hunt for the plane's black boxes is now a race against time, since the batteries on the recorders will likely go dead in a few days.
Malaysia Airlines Flight 370 vanished on March 8. The plane was carrying 239 people on a flight from Kuala Lumpur to Beijing.