The tunnel network running beneath the city was originally the channeled bed of the Guanajuato River, built to redirect floodwaters away from the low-lying historic center. After a dam was completed upstream, the river channel was repurposed and gradually expanded into a road system that now forms the backbone of the city's traffic flow. What was once purely functional infrastructure became one of the most distinctive features of the city, threading drivers through vaulted passages that feel more like a cathedral crypt than a commuter route.

Late afternoon is a lovely time to experience the tunnels in person, when the contrast between the warm electric glow inside and the fading daylight at each entrance is at its most dramatic.