Larry Stone, chief scientist at Reston, Virginia-based consultant Metron Inc., has tracked missing aircraft and ships for half a century. Stone mapped out the resting place of Air France Flight 447, which was found two years after plummeting into the Atlantic Ocean with 228 people aboard in 2009.

The location, size and characteristics of the underwater search for MH370 make it the toughest he’s ever seen. “I wouldn’t be surprised if they didn’t find it,” Stone said.

The waters in the search area are up to 6 kilometers (4 miles) deep and peppered with trenches and submerged peaks. Last month, a towed sonar vehicle collided with a volcano rising 2,200 meters (7,200 feet) from the seabed. The device was severed and sank to the bottom