WELLINGTON, New Zealand — Two miles beneath the sea surface where satellites and planes are looking for debris from the missing Malaysian jet, the ocean floor is cold, dark, covered in a squishy muck of dead plankton and — in a potential break for the search — mostly flat.
The troubling exception is a steep, rocky drop ending in a deep trench.
The seafloor in this swath of the Indian Ocean is dominated by an underwater plateau known as Broken Ridge, where the geography would probably not hinder efforts to find the main body of the jet, which disappeared with 239 people on board three weeks ago, according to seabed experts who have studied the area.
Australian officials on Friday moved the search to an area 680 miles to the northeast of a previous zone as the mystery of Malaysia Airlines Flight 370 continued to confound.
There is no guarantee that the jet crashed into the new search area. Planes that have searched for it for two days have spotted objects of various colors and sizes, but none of the items scooped by ships has been confirmed to be related to the plane.
The zone is huge: 123,000 square miles, about the size of Poland. But it is closer to land than the previous search zone, its weather is more hospitable — and Broken Ridge sounds a lot craggier than it really is.
The deepest part is believed to be 19,000 feet, within the range of American black box ping locators on an Australian ship that is expected to arrive in the area in three or four days.
Formed about 100 million years ago by volcanic activity, the ridge was once above water. Pulled under by the spreading of the ocean floor, now it is like a large underwater plain, gently sloping from as shallow as 2,625 feet to 9,843 feet deep.
Much of Broken Ridge is covered in sediment called foraminiferal ooze, made of plankton that died, settled, and was compacted by the tremendous pressure from the water above.
‘‘Think like it’s been snowing there for tens of millions of years,’’ said William Sager, a professor of marine geophysics at the University of Houston in Texas.
In places, the layer is up to half a mile deep. But if the fuselage of the Boeing 777 did fall on to Broken Ridge, it would not sink much into the muck.
‘‘The surface would be soft, it would squeeze between your toes, but it’s not so soft that you would disappear like snow,’’ Sager said. ‘‘Something big like pieces of an airplane — it’s going to be sitting on the surface.’’
Searchers will be hoping that if the latest area turns out to be where the plane crashed, the fuselage did not go down on the southern edge of Broken Ridge. That’s where the ocean floor drops precipitously — more than 2½ miles in places, according to Robin Beaman, a marine geologist at Australia’s James Cook University. It’s not a sheer cliff, more like a very steep hill that a car would struggle to drive up.
The trench’s rocky crags and crannies would make it difficult for ships using instruments such as side-scanning sonar or multibeam echo sounders to distinguish any debris from the crevices.
Searchers will especially be hoping to locate the jet’s two ‘‘black boxes,’’ which recorded sounds in the cockpit and data on the plane’s performance and flight path that could help reconstruct why it diverted sharply west from its overnight flight from Kuala Lumpur, Malaysia, to Beijing on March 8.