7. Lake Baikal
Considered the world's oldest and deepest freshwater lake — at its deepest, it plunges more than a mile — Baikal contains somewhere around 20 percent of all the fresh water on the planet. More than 300 streams and rivers flow into the lake, though only one, the Angara, heads the other way. It is home to the Baikal seal, the only freshwater seal in the world.
Baikal was formed some 20-25 million years ago and sits in a rift in the continent (like Tanganyika in Africa) where, if scientists are correct, the tectonic plates will eventually split — not tomorrow, but some day — and form separate land masses.