Unlocks the Military Engineer ability to construct a Mountain Tunnel.

Acts as a movement portal on a mountain range, allowing units to move into it and exit from another portal at the cost of 2 Movement. Trade Routes traveling through it can multiply the Gold they get from districts at their destination. Can only be built on an adjacent Mountain tile. Cannot be pillaged or removed.

The very first tunnels date back thousands of years. These ancient underground routes covered a range of purposes from transporting water to providing tactical advantages to troops. Despite the challenges of this sort of excavation at the time, the Crypta Neapolitana tunnel near Naples, dating to 37 BC, measures 700 meters in length and nearly 10 meters high. It was also completed with ventilation shafts, and was still used as a roadway until the early 1900s.

Tunnel construction really began to boom as railroads became a popular means of transport, and brought with them very specific needs. The first railroad tunnel was completed in the United States in 1834 by means of black powder blasting at a rate of 18 inches per day. Today, tunnels are commonplace throughout the world as part of public transit systems.