Caspian Gate: part of the Elburz mountains.
The Caspian Gate is mentioned in several ancient sources as a mountain pass on the road between Rhagae (more or less identical to modern Tehran) and Hecatompylos, the capital of Parthia, south of modern Damghan. The Caspian Gate is almost certainly identical with the pass between modern Eyvanekey and Garmsar.
The road is very ancient indeed: this is the course of the Silk road. Today, there is a crowded highway that connects Tehran to Mashad (which makes it hard to leave a car to take photos).
The Caspian Gate was once the border between Media and Parthia. During the war against the Macedonian invader Alexander the Great, the last Achaemenid king Darius III Codomannus selected this place as his final stand (summer 330). The pass was hard to overturn: to the south is the desert, to the north is the Elburz mountain range. It is an excellent place that can be defended by a small army. Nevertheless, Darius' courtiers considered it better to dethrone their king.
Although the terrain looks dry, there are several artesian wells, which bring the water from the Caspian Sea area to the south. As you can see on the picture of this bizarre landscape, there are many small mountain gullies in the spring.