Finding an aircraft on a seabed several kilometres down is a search problem, and the way it is solved is by repeatedly shrinking the area rather than by looking harder. There are four stages, each using a different technology and each reducing the problem by roughly an order of magnitude.
Stage one: define the box
The starting area comes from the aircraft's last known position, its likely track, its fuel state and its performance. For AF447 this was straightforward — the aircraft had transmitted ACARS messages during the event, so there was a position and a time. For MH370 it required deriving an arc from satellite handshakes never intended to locate anything.
This stage produces an area measured in thousands or tens of thousands of square kilometres.
Stage two: drift analysis
Anything that floats — debris, cushions, life jackets, bodies — is found on the surface, sometimes days later and hundreds of kilometres from the impact point. Oceanographers run the currents and winds backwards from each recovery, building a probability field of where the material entered the water.
For AF447 this was done within days. For MH370 the first debris appeared sixteen months later, on the other side of an ocean, which made the backward modelling far coarser.
Stage three: listen
If the search reaches the area within the beacon's life, vessels tow hydrophones through the box listening for the recorder pinger. This is the fastest way to find the aircraft, and it is the stage that failed in both AF447 and MH370 for the same reason: the box was not defined in time.
Stage four: survey the seabed
When the beacons are silent, the only remaining method is to map the seabed and look for something that does not belong.
Modern searches use autonomous underwater vehicles carrying side-scan sonar. Each vehicle descends to a set height above the bottom, flies a programmed lawnmower pattern for a day or more, surfaces and uploads its sonar imagery. Analysts examine it for the geometric shapes and acoustic shadows that distinguish wreckage from rock.
This is slow, expensive and thorough. AF447's wreckage was found this way in 2011, in an area that had been searched and cleared in an earlier phase — a reminder that a negative result at this stage means "not detected", not "not there".
What makes it hard
Depth limits which vehicles can be used and how long each dive takes. Terrain matters enormously: a debris field on a flat abyssal plain stands out, while the same field among ridges, canyons or volcanic rock can be acoustically invisible. And much of the deep ocean has never been mapped at useful resolution, so searchers are often producing the first detailed chart of the area as they go.
The MH370 search produced a bathymetric survey of a large area of the southern Indian Ocean that is now a scientific resource in its own right. It did not find the aircraft.