At first glance, it's not that much of a lake. Less than 400 feet in diameter, it's very close to a perfect circle. It lies at the bottom of a steep depression, completely surrounded by cliffs close to 100 feet high. The water is very dark - almost black, and much of the surface is choked with pond weed.
Talk to the park ranger and you'll find that the lake is fed from underground volcanic vents, and contains lethal levels of arsenic and other poisons. There is also no oxygen in the water, so fish cannot survive more than a few minutes in it. Fascinatingly, the lake has no known bottom. It simply gets denser and increasingly saturated with sand and mud until further exploration is impossible. (The deepest probes have made it to 124 feet before being stopped by the suspended sand, without finding any bottom. Some geologists estimate the lake may be 2000 feet deep!) I imagine that it's similar to what one would find on the gas giants in the outer solar system, which have no solid surface, but just keep getting denser and denser as you go down until you're stopped dead..
But that's just the first course. Now for the main event. Montezuma's Well boasts a unique ecosystem of species which exist nowhere else on the entire planet. There's a completely independent food chain of life beginning with diatoms (a species of algae), the Montezuma Well springsnail, a water scorpion, the Hyalella montezuma amphipod, and (at the top of the chain) the Motobdella montezuma leech, along with the lake's utterly unique pond weed with 80 foot long stems that grows nowhere else in the world. All of these species would perish if removed from Montezuma's Well - they're perfectly adapted to its poisonous environment. And likewise, no life from outside the well can live in its waters for longer than a few minutes. The life within Montezuma's Well is essentially independent of all other life on Earth!
This amazing site got me to thinking about possible life on other planets. Perhaps we're too accustomed to the ubiquity of life covering nearly every inch of our own world. But is it possible that we'll find the equivalent of a Montezuma's Well on Mars, or even the Moon? A micro-environment only a few hundred yards across sustaining a vigorous ecosphere of utterly bizarre lifeforms? 99.9999% of Mars could well be deader than a doornail, while one small crater might be absolutely bursting with life. After all, as far as the denizens of Montezuma's Well are concerned, the rest of Planet Earth might as well be dead.
Ice-filled Korolev Crater on Mars