The Caspian Depression or Pricaspian/Peri-Caspian Depression/Lowland is a low-lying flatland region encompassing the northern part of the Caspian Sea, the largest enclosed body of water on Earth. It is the larger northern part of the wider Aral-Caspian Depression around the Aral and Caspian Seas.
Caspian Depression and north Caspian Sea from space. NASA photo