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Investigators have detected possible underwater signals over the past three days that may be linked to the missing Malaysia Airlines jet's black boxes. But the battery-powered beacons may have already died, and searchers haven't picked up any more pings with their sophisticated listening devices.

The scale of the challenge is overwhelming. The pinger locator towed by the Australian Navy vessel Ocean Shield is working at a depth of 4,600 feet. But the signals were detected at roughly 15,000 feet below the water's surface, the maximum-known depth of the ocean floor below the ship.

The sheer depth of the possible location of Flight 370 poses a complex challenge for searchers. Watch the animation below to learn why: