A map of the Caspian Sea pointing out Dagestan. (TV: The Waters of Mars)
The Caspian Sea was the largest inland sea on Earth. Dagestan was located by the Caspian Sea. (TV: The Waters of Mars)
Alternative timeline
In an alternative timeline during the Year That Never Was, the Saxon Master poisoned the sea. (TV: Last of the Time Lords) Martha Jones later visited it during her travels around the globe. (PROSE: The Story of Martha)