How can a jetliner more than 200 feet long be missing four days? It has happened before.
Under normal conditions, a commercial plane's transponder sends out information that allows radar stations to determine its precise location, altitude and speed.
But there are no radar stations in the middle of large bodies of water, leaving gaps in radar detection. To make matters worse, for an unknown reason, Malaysia Airlines Flight 370's transponder apparently stopped working. For now, those two factors make it difficult to know what happened to the plane, which vanished Saturday morning with 239 people on board.
"We assume the plane is crashed, but [there is] not a bit of evidence, not a mayday, nothing," said David Gallo, an oceanographer.
Gallo knows the challenges of finding a plane at sea. He helped to lead the search for Air France Flight 447, which crashed in the Atlantic Ocean in 2009.
"You've got to deal with currents, you've got to deal with visibility, you've got to deal with things like wind and waves if the ocean acts up," Gallo said. "It's a much more difficult task than finding something on land."
Dozens of ships, including two U.S. Navy destroyers, are using what is known as a "creeping line search," following a precise pattern of equally spaced parallel lines.
If they find evidence that could be connected to a crash, a ship can switch to an "expanding square search," working its way outward with a tight series of turns, looking for additional evidence.
The ships drag sonar devices to listen for pings from the plane's black boxes, just as they did in 1996 in the search for TWA Flight 800 off New York's Long Island.
"I never have a lot of faith in the pingers underwater in the ocean, that they are going to be heard," Gallo said. "In some cases I think you could be very close to them and because of the structure of the water or the topography around hills and valleys and what not, you may not hear the pinger at all."
Back in 2009, it took five days to find the first evidence of that Air France plane that crashed in the Atlantic, and it took two years to find the black boxes.