It is Lake Baikal - located in Siberia, Russia.
It is approximately 5,500 ft (1,700 meters) deep - which is equivalent to roughly 4 Empire State buildings stacked on top of one another, and holds 20% of the world's fresh water reserves.
It is also the world's oldest lake, at over 25 million years old, and is home to a wealth of rare species of flora and fauna.
During the winter months, the lake freezes over and you can walk across it, but at your own risk -- severe cold winds blow across the frozen, flat landscape.
Several famous people, including the Hollywood movie director James Cameron, have taken a specially designed submarine to explore the bottom of the lake, which is supposedly very flat. Cameron got some of the inspirations for his movie Avatar from the scenes he saw in Lake Baikal!