BBC Copyright: BBC

Oceanographer Dr Simon Boxall tells the BBC that Flight MS804 went down in a very difficult part of the Mediterranean to search.

"I think the assumption is that this is a shallow water area but it's not, this part of the Mediterranean is really quite deep," he said.

"We only have limited information about the crash location, but if it is where this debris has been found then it sits on a boundary where the ocean floor is more like the Alps, very rugged and contoured.

"That area borders a large, deep plain about 3km down."

Dr Boxall said an aircraft such as the A320 could easily "slip through the cracks" of the ocean floor in the area.

"This is a very soft sediment area and wreckage could sink very quickly, so they need to find it fast," he said.

The ocean floor ridges could also act to block signals from the black box pinger, and sonar from vessels searching for the wreckage, he said.

"Having thought this was a relatively simple one, it could actually be very difficult."