The area of the ocean floor being scanned for remnants of MH370 is so far underwater that a robotic submarine used by the Navy for deep-water tasks had to head back to the surface after reaching its maximum depth on Monday's search mission. From the AP:
Search crews sent the U.S. Navy's Bluefin 21 into the depths Monday to begin scouring the seabed for the missing Malaysia Airlines Boeing 777 after failing for six days to detect any new signals believed to be coming from its black boxes.
But the 16-hour mission was cut short when the unmanned sub, which is programmed to hover 30 meters (100 feet) above the seabed, entered a patch that was deeper than its maximum depth of 4,500 meters (15,000 feet), the search coordination center and the U.S. Navy said.
There are other submersibles that can go deeper—James Cameron has been in person to the deepest goddang part of the ocean, 36,000 feet below the surface—but the search team hasn't yet found one that's available.