A few weeks ago I wrote about the way that spacecraft regularly take detailed pictures of the entire surface of the planet. I noted that we now can, with satellite assistance, determine where everything is to within a few meters. Anything like Arthur Conan Doyle's novel, "The Lost World," with its great undiscovered plateau in South America populated by life forms long believed to be extinct, is no longer plausible or even possible.

What I didn't say is that my statement is true only if we are referring to the surface of the planet. The things we might find within the seas and deep oceans of the world, and where we might find them, is another matter entirely. And Earth is a water world. Seas and oceans cover almost three-quarters of it.

Why are we so ignorant of so much of the world that we live in?

In a few words, we are ignorant because underwater exploration is enormously difficult. Space exploration is generally regarded as hard, something at the limit of our technology, and I'm not going to argue that it is easy. However, I do believe that "outer space" is, in almost every way, simpler to explore than "hydrospace," the underwater world.

Consider some of the key variables. First, how can humans survive in either environment? Out in space, surrounded by vacuum, a spaceship containing air at normal atmospheric pressure must be able to withstand an (outward) force of about 14 pounds per square inch on its hull. Diving into the ocean, a submersible feels that much inward pressure when it is just 34 feet below the surface. Cornelius Van Drebbel, a Dutchman, built the first underwater boat in 1620 and it was powered by an intrepid group of six pairs of oarsmen. It went as deep as 15 feet in the River Thames, and this was (rightly) regarded as a considerable feat. It could not have gone much deeper without the collapse of the hull.

The depth of the seabed in places, however, is more than 30,000 feet. The pressure there is monstrous, a thousand atmospheres, the water is only a few degrees above freezing, and the darkness is total. Humans have descended to these depths, just a few times, and observed a tiny area of the dark ocean floor. The old argument, manned versus unmanned, may still continue for space projects, but it is not an issue in the deepest oceans. If you can do a job with an unmanned submersible, remotely controlled from a ship on the surface, that is the way you do it.

Seeing things underwater is another big problem. We can turn our telescopes and radio telescopes upward and probe galaxies out toward the edge of the visible universe, more than ten billion light-years away. Looking down, we see at best a few hundred feet. Natural sunlight is extinguished totally at 600 feet and explorers, whether in person or through a remote submersible, must carry along their own sources of illumination. Even then, the world will fade and dim only 100 feet away from them.

Since we cannot see the bottom of the deep ocean from the surface, and since it is extremely difficult to go there, mapping the seabed must rely on other methods. The standard techniques we use from orbiting satellites just won't work. Radar and radio waves, unless at very long wavelengths, hardly penetrate water at all. The seafloor can be mapped using reflected sound waves, but such imaging sonar produces pictures of long, thin strips of the seabed, which must then be painstakingly stitched together. The budget for such underwater work is small unless there are strategic reasons for needing detailed maps.

Depth, darkness, extreme pressure; it sounds like a totally inhospitable place to live. But there is life, even in the deepest marine trenches. And there are surprises. It is 62 years since the coelacanth, a fish "extinct" for 60 million years, was caught alive off the coast of South Africa. It was swimming about 250 feet down, where available sunlight can be no more than a wan glimmer in even the clearest water.

Presumably it was hunting, because the coelacanth, about five feet long and weighing over 100 pounds, is carnivorous. Long before it was discovered, however, and ever since, there has been talk of much bigger unknown sea creatures, sometimes briefly glimpsed, sometimes washed up dead onto beaches. Within the past three years, an expedition went off to seek the giant squid in waters north of Australia. They didn't find it, but whales have been caught bearing large sucker marks that give evidence of the squid's existence. That suggests tremendous hidden battles, far beneath the surface. Even if we allow that every fisherman talks about "the one that got away," the oceans force us to admit the possibility that other life forms, perhaps ones believed to no longer exist, are lurking in the darkness. If the coelacanth from 60 million years ago is still with us, why not the plesiosaurus, a 30-foot marine reptile thought to have become extinct at about the same time?

We are poorly equipped to search for such long-lost forms, but others are not so restricted. Seals sometimes descend a quarter of a mile below the surface. Presumably they, like the coelacanth, are hunting. Whales can dive a mile and more, and during their descent they experience pressure changes of more than a hundred atmospheres. How they can stand something that would be certain death to any human still is not fully understood. Nor do we understand why they go there, or know what they may encounter a mile down.

Today's real "lost world" may still be there, far below the ocean surface. Humans, however, may be far from the best-equipped creatures to explore it.

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