The Ogarrio Tunnel is the ceremonial entrance to Real de Catorce: a narrow, one-lane passage through the mountain that makes the town feel deliberately hidden. Traffic is controlled from each end, so you may wait before the slow drive in.
It was opened in 1901 to connect the mining town with the outside world. Today it is less a piece of infrastructure than part of the travel experience, especially if you arrive in a shared truck or old jeep with the tunnel lights sliding over the stone.