What is the origin of the Caspian Sea basin?

Today, this reservoir does not have an exact definition of its status – whether it is a sea or a lake. The Caspian Sea is not connected with any ocean, and the main criterion of the sea is just such a connection. However, this lake used to be a real sea and was related to the ocean, but several million years ago, the consequence of the uplift of the relief was separated from the ocean waters. In addition, this reservoir was also formed in the former ocean trench. Currently, the reservoir is closed. It is quite deep – in some places the depths reach thousands of meters.