A critical search is now underway to find the missing Malaysia Airlines jetliner’s black box, which will soon run out of battery power and may hold the key to understanding what happened to the Boeing 777.

Two ships equipped to search under water are scanning a section of the Indian Ocean today to look for the black box, according to Australia’s Joint Agency Coordination Centre.

The boxes are nearly indestructable and record crucial data that would help investigators piece together what happened to the plane. The data recorders send a transmission (or ping) that can be detected by the ships. The boxes, however, are only designed to last 30 days before running out of battery power. This means the black box on flight 370 may cease transmissions in the next few days, according to Time.

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The lack of any confirmed debris combined with the black box's rapidly depleting battery leaves precious few options. The estimated life of the power supply is 30 days, or April 7.

An Australian navy ship called Ocean Shield, which is dragging a towed pinger locator from the US Navy, and the British navy’s HMS Echo are conducting today’s underwater search for emissions from the plane’s black box.

This is the first sub-surface search that has taken place in the hunt for the Malaysia jetliner’s black boxes.

“No hard evidence has been found to date so we have made the decision to search a sub-surface area on which the analysis has predicted MH370 is likely to have flown,” said Australian Commodore Peter Leavy. “The search using sub-surface equipment needs to be methodical and carefully executed in order to effectively detect the faint signal of the pinger.”

The ships will move at a reduced speed in order to search at depths of 3,000 meters (about 9,800 feet) or more said Leavy, who is commander of Australia’s Joint Task force 658, which is coordinating military contributions for the search. The pinger locator from the US Navy should be able to pick up signals from an even greater depth, the Associated Press reports.

Because the U.S. Navy's pinger locator can pick up black box signals up to a depth of 6,100 meters (20,000 feet), it should be able to hear the devices even if they are lying in the deepest part of the search zone -- about 5,800 meters (19,000 feet) below the surface. But that's only if the locator gets within range of the black boxes -- a tough task, given the size of the search area and the fact the pinger locator must be dragged slowly through the water at just 1 to 5 knots, or 1 to 6 miles per hour.

Map of the current search area for flight 370.
Australia’s Joint Agency Coordination Centre

A total of 14 aircrafts and 11 ships are involved in today’s search, which is taking place in a 217,000-square-kilometer area (about 84,000-square-miles) that is 1,700 kilometers (about 1,050 miles) north west of Perth, Australia (which is on the west coast of the country). The search will focus on three portions of the larger search area.

It has been almost four weeks since Malaysia Airlines flight 370 lost contact during a flight from Kuala Lumpur to Beijing. There were 12 crews members and 227 passengers on board the jetliner.

Since the plane disappeared, the search area has shifted based off of new radar data analysis, but no wreckage has been found yet.

Video: How a black box works

What we know about missing Malaysia Airlines flight 370

The passengers of Malaysia Airlines flight 370