Mike Purcell, a senior engineer at the Woods Hole Oceanographic Institution in Massachusetts, who led two underwater search expeditions for the wreck of Air France Flight 447 in the Atlantic in 2010 and 2011, said that the current search zone for Flight 370 was far more remote, with rougher seas and higher winds. “That can slow down your progress considerably, because it makes it more difficult to operate, to get the vehicles in and out of the water,” he said. On the positive side, he noted, the sea floor in the southern Indian Ocean is relatively flat.

Military submarines can do little to help. Their sophisticated equipment for detecting signs of surface ships or other submarines is meant to be used mainly within a few hundred feet of the surface, and has only a limited ability to detect pings from the ocean floor far below, transmitted through water of varying densities and temperatures. Towed submersible devices, on the other hand, can operate at depths of 14,000 feet or more.

Even if the plane’s black boxes are recovered, they may not tell investigators enough to explain what happened to Flight 370, which stopped communicating with ground controllers and veered radically off course about 40 minutes after takeoff. The plane flew on for at least seven more hours, but its cockpit voice recorder would have stored only the two last hours of sound in the cockpit. The separate data recorder would have technical information from throughout the flight, but that data may not reveal the intentions of whoever was in the cockpit.

For now, aircraft from Australia and other countries have been scouring an area the size of the western and southwestern United States where the plane is believed to have flown after it sent its last automated signals to a satellite. Because the search area is so far from land, planes are able to spend only a few hours searching at a time before they must turn back to base. So far, none of the handful of floating objects spotted by satellites in the search zone have been found again by other aircraft sent to the same area.