Almost 90 years ago, Arthur Conan Doyle took a break from writing about Sherlock Holmes and produced "The Lost World," a novel about an undiscovered plateau in South America where dinosaurs and other extinct species still thrived. It was an exciting tale of adventure, and even if at the time it was implausible, it was not totally impossible. The planet had not been fully explored and mapped. Mysteries might still lie in the deep unexplored heart of the tropics.

Today it's a different story. With satellites taking images of the Earth on a daily or even an hourly basis, some of them showing ground detail to a few feet, the idea of a large unknown plateau or a big undiscovered lake is sheer science fiction.

If you are reading along and agreeing, stop and consider this: there exists a real lake, more than half the area of Lake Erie and ten times as deep. If it were dropped into North America, it would be accepted without question as one of the Great Lakes. Yet its existence was unsuspected as recently as 30 years ago.

The lake is Lake Vostok. It is the largest of many lakes that lie within the thick Antarctic ice cap, almost two miles below the surface. The water in Lake Vostok remains liquid because of the high pressure (almost 350 times atmospheric pressure), and because of possible geothermal heating from below. There is no doubt about its area, which was mapped using the return signal from radar with a wavelength of about five meters. The maximum depth is estimated as 670 meters. Lake Erie, for comparison, has a maximum depth of about 70 meters.

There is, however, one very important unanswered question about Lake Vostok: Does it contain any form of life?

At first sight, that seems highly improbable. The lake has had no contact with the surface - even the bleak Antarctic surface - for more than a million years. The pressure is extreme, the darkness total. With no sources of light, there can be no plant life. There are no known nutrients for plants or animals.

Life for our own kind would be out of the question. We are pampered objects, able to survive without artificial aids (clothing, air-conditioning, heating) only between about the freezing point and boiling point of water, and comfortable only within half that range of temperatures.

Other creatures, however, are less choosy. What we are finding, year by year, is that living organisms survive in environments that are, by our own standards, totally intolerable. Life thrives in the deepest of the deep oceans, more than six miles down, where the pressure on every square inch of a creature's surface is more than six tons. There is no light. Most of the animals in the deep marine trenches are supported by the thin drizzle of organic material that rains down from the surface.

Most, but not all. Some forms of life are not light-powered - making use of sunlight to perform photosynthesis and build organic compounds from simple molecules - but chemical-powered, deriving the energy they need from sulfur and methane. Bacteria survive near deep-water sea vents known as "black smokers," places that match our own visions of Hell. The water around a black smoker is far hotter than the usual boiling point (high pressure prevents water from boiling, just as it prevents it from freezing), and what issues from a black smoker is so high in sulfur and other minerals that we would find it poisonous. Other life forms feed on the bacteria, to create great colonies of tubeworms and strange crustaceans.

Other anaerobic bacteria, thriving in methane gas and poisoned by oxygen, live at high temperatures underground. No one knows how far down, though there are suggestions that some form of bacterial life may endure five miles and more beneath the ground, as deep within the earth as the deepest of the marine trenches.

At the other extreme, organisms survive in the Antarctic, living and breeding in small lenses of liquid water that form just below the surface when the weak Antarctic sun is able to melt the ice. They return to a dormant condition through the long Antarctic winter. Lichens cling to the bare rock of high mountains, well above the snow line, exposed to the sun's fierce ultra-violet radiation from which we are normally shielded by the bulk of the atmosphere.

Life seems able to adapt to almost anything. Heat, cold, pressure, radiation, darkness. On the basis of what we have seen so far, it will be no great surprise if living creatures are found in the icy depths of Lake Vostok. The known forms and their environments are so diverse one wonders if these creatures living on the edge - collectively known as "extremophiles" - have anything in common.

They do. So far as we know, every living thing, no matter how big or how small, no matter how or where it lives, no matter if it is plant or animal or another of the kingdoms of life, shares one characteristic: everything on earth, or under the earth, or deep within the oceans of earth, depends on DNA and RNA in order to reproduce.

If we ever discover anything without this DNA/RNA dependence, then we can truly claim to have found aliens among us.

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