Most lakes are small enough to see across. The Caspian is so huge that from one side, you cannot see the other - just water meeting the sky like an ocean. It is about 1,200 kilometres from top to bottom, which is roughly the distance from London to the south of France.
The water in the Caspian is a little salty - about a third as salty as the sea - but not as salty as the proper ocean. That is because rivers from all around carry fresh water into it, and there is no current pulling salt in from outside. It is a strange in-between place that has its own kinds of fish and animals that don't live anywhere else.
The capital city of Azerbaijan, Baku, sits on the Caspian's western coast. The Baku Boulevard is a long, palm-lined walkway by the water where families stroll on warm evenings. Children ride scooters, eat ice cream and watch the sun set across the sea.
Lots of natural gas and oil sit under the seabed of the Caspian. Long ago, oil even bubbled up to the surface on its own - travellers on the Silk Road wrote about black liquid floating on the water and burning. Today, careful work keeps the sea clean while the country shares its energy with the world.