whereas far as we know the bluefin-21 is now about seven hours into mission number two. it was around this time yesterday that the subhad to abruptly abort the mission after completing only a third of what it had hoped to do. will it be able to continue a full scan as it continues to search for flight 370? tonight, the robot sub that may be the best hope of finding malaysia airlines flight 370 is trying to finish its second mission after its first was abruptly cut short. the device known as bluefin-21 did not find the data recorders or wreckage and was forced to return to the surface less than a third of the way into its mission monday. the sub found itself in water too deep to operate. >> this is an area that is new to man. >> reporter: the extreme depth took searchers by surprise and