By Marlene Cimons
Lake Baikal is the world's oldest and deepest lake. It's at least 20 million years old, and roughly a mile deep at its lowest point. The Siberian lake contains holds more water than all the North American Great Lakes combined, what amounts to more than one-fifth of all the water found in lakes, swamps and rivers. It was formed by the shifting of tectonic plates, which created a valley that filled with water. That shift continues today at a rate of around 1 to 2 centimeters year, meaning the world's biggest lake is only getting bigger.