Silent black box leaves MH370 searches with a 'shot in the dark'(2)

Robertson said multi-beam and side-scan sonars determine the depth of the ocean and the thickness of different types of sediment, such as mud or sand.

"But the depth of the ocean is not well known. We know the Moon and Mars better than we know the ocean floor."

Search coordination from the ADV Ocean Shield has limits in an area almost totally unmapped.

And that vast ocean has turned out to be deeper than expected.

According to Robertson, "They discontinued using the autonomous vehicle, since it needs to be within 30m of the ocean bottom to work properly and that now appears to be beyond its depth range."

At such depths, pressure becomes a factor, and technology is yet to be created that can endure long-time operation.

Pressure increases by about 1 atmosphere of pressure for every 10m descent (one atmosphere is about 1 kg per square centimeter or 14.7 lb per square inch). At 4,500 meters there would be 450 more atmospheres of pressure. Equivalent to about 465 kg of pressure per square centimeter, according to Robertson's calculations.

It's estimated the search area as roughly 75,000 square kilometers, larger than the nation of Belgium.

Frustration and anxiety have marked an extraordinary international effort that has spanned across the Asia-Pacific over 40 days.

Even last month, putting some perspective on the geographical challenges, the Vice Chief of the Australian Defense Force (ADF), Air Marshal Mark Binskin told reporters, "We are not searching for a needle in a haystack."

"We are trying to define where the haystack is."