This, however, didn't entirely solve the flooding problems and it wasn't until they dammed the Guanajuato river in the 1960s that the flooding was finally under control (this is different from the Presa de la Olla on the southeast side of town). Once the dam was built and the tunnels weren't needed for the river anymore, they updated them for traffic. And that make Guanajuato a very interesting place to drive around. It looks like they might have built the first tunnel specifically designed for cars in the 1950s.
I'd read about the tunnels before we went there, but I hadn't realized people could walk through them. There are stairs for pedestrians to access the tunnels at different places, and you can park in some of the tunnels, which we did because who can pass up a chance to park in a tunnel?
This is probably a good place to mention that it's a much better idea to buy a Guia Roji map in town (we bought one at Oxxo) than to rely on the awful standard tourist map we were given on our last day there. The first wasn't perfect (because I don't think it's possible to create a perfect map of Guanajuato), but after navigating the city on foot and in the car with that one, I cannot see how anyone could possibly find the tourist map helpful. I saw an older couple trying to use it and wished we didn't still need our map because they were going to spend their day lost. I should have told them to get something better.