Area: 15,690 Sq. km.
Population: either 'none' or 'very interesting,' but probably nothing bigger than microbes
Volume: 5,400 cu. km.
Maximum depth: 1,000 m
Rank among world's largest lakes (area): 15
Rank among world's largest lakes (volume): 7
Forty million years ago, Antarctica was a rather balmier place than it is today. It was still attached to South America and Australia, a tectonic configuration that diverted warm oceanic currents toward the south pole, keeping the continent warm and ice-free, even lush. But South America and Australia wouldn't stand for this; they sailed off towards the equator, stranding Antarctica in a ring of cold ocean (the jerks). It wasn't long before a crown of ice began to bloom at the south pole; it would continue to grow until it smothered nearly the entire landmass, crowding out whatever flora and fauna had once made the place home, and even depressing the continent itself; much of Antarctica - the ground way below all that ice - has been dunked below sea level by the incredible weight pressing down on it.
Among the features of the Antarctic landscape which were rolled over by the advancing ice was a lake - one of the largest in the world, in fact: 250 km long and 50 km wide. But rather than gouging out the lakebed or freezing it to the bottom, the glaciers built on top of the lake. By 500,000 to as many as 25 million years ago, the lake was completely sealed off from the external environment, entombed in utter darkness. But it's still a liquid body of water; the water temperature is -3C, but the intense pressure from the weight of the ice keeps it from freezing.
The lake wasn't discovered until the 1990s, when some Russians happened to be drilling the world's deepest ice core directly above the lake. They came within a hundred yards of piercing the lake's surface before their colleagues persuaded them to quit the effort. (The drilling hole was filled with a slurry of chemicals to keep it from re-freezing: to have pierced the surface would have been to pollute the world's most uncontaminated environment.)
The environment of Lake Vostok is utterly unique, and it raises an intriguing question: is anything alive down there? If anything is, it would represent a genetic cul-de-sac cut off from the rest of life on Earth for perhaps millions of years, evolving in an environment that for all intents and purposes is an alien world. And indeed, life down there would have to have evolved: another effect of the extreme pressure in Lake Vostok is a level of oxygen which would be lethal to anything living on the surface. But life is stubborn. Once it insinuates itself into an environment it tends to stick around, and living stuff has been found in lots of weird places, including in the microscopic crevasses between snow crystals at the south pole; and indeed, nearly 12,000 feet below the surface of the ice directly above Lake Vostok. As Damn Interesting says:
It is not unreasonable to suggest that cold-tolerant creatures could thrive in the waters of Lake Vostok, overcoming the oxygen saturation with extraordinary natural antioxidants. But millions of years of evolutionary isolation in an extreme environment may have created some truly bizarre organisms. This notion is supported by the ice samples drawn from the ice just above Lake Vostok, where some unusual and unidentifiable microbial fossils have been found. But the possibility that they are merely contaminates has not yet been completely ruled out.It would be interesting to take a peek down there, though hard to do without contaminating the most pristine body of water on Earth - and contamination in this case might mean the introduction of a few microbes which could wreak havoc with whatever ecosystem might exist down there. The Jet Propulsion Lab has worked on plans for a "cryobot" - a probe that would melt its way down to the lake like an atomic gopher; it would then swim around and see if it could find anything lively. And if by chance it did, then our conception of the sorts of places that life might call home will, once again, have expanded.
Take note, Europa.