(CNN) -- Every day the whereabouts of Malaysia Airlines Flight 370 remain a mystery, the search area grows, complicating rescue efforts.
"Crucial time is passing," David Gallo, with the Woods Hole Oceanographic Institution, told CNN's Wolf Blitzer on Monday. "That search area -- that haystack -- is getting bigger and bigger and bigger."
Nearly three dozen aircraft and 40 ships from 10 countries have so far failed to find any sign of the aircraft, which took off from Kuala Lumpur shortly before 1 a.m. Saturday (noon Friday ET).
The Boeing 777-200ER, carrying 227 passengers and 12 crew members, went missing while flying to Beijing.
Gallo described what will happen once some debris from the aircraft is found, though he stressed there's still no evidence the plane hit the water.
"Once a piece of the debris is found -- if it did impact on the water -- then you've got to backtrack that debris to try to find the 'X marks the spot' on where the plane actually hit the water, because that would be the center of the haystack.
"And in that haystack you're trying to find bits of that needle -- in fact, in the case of the flight data recorders, you're looking for a tiny little bit of that needle," he said.
Rescue officials have expanded the search area.
"What I'm seeing here is clearly they have no idea," said CNN aviation correspondent Richard Quest. "They know roughly the area, but even there they are starting to scrabble around as to -- was it going in this direction? Had it turned round?"
The newly expanded search area encompasses a larger portion of the Gulf of Thailand between Malaysia and Vietnam, according to Azharuddin Abdul Rahman, director general of the Malaysian Civil Aviation Department.
Quest described the search as "extremely painstaking work," suggesting a grid would have been drawn over the ocean and that teams are combing the area, bit by bit.
"Let's not be fooled into thinking this isn't a vast swath of water. That area between Malaysia and Vietnam is still several hundred miles. That's an enormous amount of water," he said.
Although the work is challenging, Quest is confident Malaysia Airlines Flight 370 will be found.
"It's not hopeless by any means. They will find it.," he said. "They have to. They have to know what happened."
'Still a mystery'
Gallo, of the Woods Hole Oceanographic Institution, speaks from experience.
He helped lead the search for the recorders of Air France Flight 447, which crashed into the Atlantic Ocean in 2009.
The Air France flight was en route from Rio De Janeiro to Paris when communications ended suddenly from the Airbus A330, another state-of-the-art aircraft.
It took four searches over the course of nearly two years to locate the bulk of flight 447's wreckage and the majority of the 228 bodies in a mountain range deep under the ocean. It took even longer to find the cause of the disaster.
In 2011, the aircraft's voice recorder and flight data recorder were recovered from the ocean floor after an extensive search using miniature submersible vehicles.
"In this case, I thought for sure -- in a highly-trafficked area where there's lots of air traffic, lots of ship traffic, not far from shore -- that for sure this would be a more rapid finding of some remnants of the plane -- but nothing," Gallo told CNN's Blitzer, comparing the Malaysia Airlines and Air France flights.
Cmdr. William Marks of the U.S. 7th Fleet similarly spoke to Blitzer on Monday.
"We are looking over a -- now -- a very large search area in the Gulf of Thailand, and then yesterday, an area northwest of the Strait of Malacca. But as the hours go by, that area keeps getting bigger and bigger when you account for currents and the wind," he said, while aboard the USS Blue Ridge, which is assisting in the search.
Marks continued: "It's a very large search area, but still a mystery -- still a lot of question marks."
CNN's Saeed Ahmed contributed to this report.