The city was originally built over the Guanajuato River, which flowed through tunnels underneath the city. However, after years of raising buildings to accommodate repeated flooding, in the mid-twentieth century, engineers built a dam and redirected the river into underground caverns. The tunnels were lit and paved with cobblestones for automobile traffic, and this underground road network carries the majority of cars driving through the city today. It is one of the most notable features of the city.
The city played a major role in the Mexican War of Independence since it is the capital of the state of Guanajuato in which Miguel Hidalgo started the independence movement. [Wiki]