The closer the search for MH370 gets to the source of its black box beeps, the further away it seems. Lost in the profound deeps, its forlorn sound seems meaningless now, in human terms at least. It is only a signifier for grief. It seems hardly believable that, in an age when a satellite can read a car number plate, we cannot locate an entire airliner.

Indeed, according to one highly placed oceanic scientist to whom I spoke recently, it is truly unbelievable: he believes, as do other independent scientists, that the Americans have known exactly where the flight crashed ever since it fell out of the skies; to reveal that they do so would be to endanger covert military information and the operation of underwater sonic arrays whose primary purpose is to detect such large objects in the oceans – in the shape of enemy submarines.

Whatever the multifarious conspiracy theories that have and will continue to grow around the news vacuum of MH370's mystery, this story speaks to our essential ignorance. These underwater mountains and trenches remain the least unexplored places on earth, in area and extent the size of China, yet almost entirely unmapped, even now. Until 1773, when Constantine John Phipps – second Baron Mulgrave, officer in the Royal Navy, Eton school friend of the explorer Joseph Banks, and the first European to properly describe the polar bear – began to sound the ocean bed, many people thought the sea was bottomless. It was just as they had once thought that the world was a flat disc over whose edges one might sail into the void or into the maw of a monster.

During his voyage towards the north pole in search of the fabled north-west passage – sailing from Deptford, in London, on the bomb vessels Racehorse and Carcass (along with a young midshipman named Horatio Nelson) – Phipps employed a lead weighted line to measure the distance between himself and the bottom of the ancient sea between Iceland and Norway. That strand of hemp linked the Enlightenment with the pre-history of the earth. It drew 683 fathoms, 1,250 metres, and for a century this northern nadir remained the profoundest known ocean.

In 1872, a hundred years after Phipps's first investigations, HMS Challenger set off on a three-and-a-half-year voyage to chart the seas and examine whatever she might trawl up from their depths. The expedition was inspired by two, perhaps conflicting, aims: pure science, and commercial communications. Aboard were five scientists with their own shipboard laboratory, an expedition artist, and a parrot that would call out at intervals: "What? Two thousand fathoms and no bottom?" As it sailed into the Pacific, Challenger discovered those depths to be greater than ever suspected.

Sailing north from New Zealand towards Japan, her scientists took a measurement of 26,850 feet (8.2 km) off the Marian Islands in the western Pacific, proving this to be the deepest of all oceans. This became known as the Challenger Deep, and its discovery shows how accurate the expedition was, being close to the lowest depth that we now know of: 36,198 feet (11 km), since recorded in the Mariana Trench.

Now, as the Australians and Chinese (but interestingly not the Americans) race to locate MH370 in similar profounds, their sonic searches echo the new Asian century, and a new arena of exploitation and power. At the same time, their efforts remind us that the ocean is a noisy place; scientists have warned that many of those bleeps and whistles picked up by their hydrophones could belong to marine mammals instead. Sound travels five times faster in the water than in the air – it's why a fin whale on one side of the Atlantic can be heard by another whale on the far side of the same ocean. And when humans first heard whales singing, audible through the hulls of their ships, they believed they were listening to ghosts; just as Marconi believed his radio signals might pick up the sounds of long-dead men who had drowned in the Atlantic. Those forlorn bleeps from 4,000 metres down in the Indian Ocean may be bringing us closer to closure in the story of the MH370. But they also remind us that this watery planet can still keep its secrets from us.