The preliminary sea-bed mapping of the area has found areas where the water depth is as shallow as 600 metres and within a couple of kilometres, it has gone to 6000 metres. It has also found undersea volcanoes. "Challenging" is the understatement of the month for this piece of seabed, and the location and recovery of any critical MH370 wreckage is going to be one of the most difficult undersea operations ever mounted.