In the alley of Cinco de Mayo Road, there's a doorway leading underground that looks to be an entrance to a subway. But in Puebla, there are no subways. It is, in fact, the tiny entrance to a recently discovered secret of the city, a tunnel system used to connect a fort with the baroque city of Puebla.

The underground tunnel system dates back to 1531, but wasn't unearthed and opened to the public until 2016. It had been covered for decades, and the archeologist who unearthed the tunnel discovered antiques in the mud. Today it operates as a museum that doubles as a citywide thoroughfare.

Many Mexican cities have legends about secret tunnels lying just beneath the streets, used during the revolution either by royalty or even during the inquisition. Grandparents would tell these folk stories to children. The discovery of the Puebla tunnel lends some credence to the folklore.

The tunnel—tall enough that a person could comfortably ride through on horseback—originates in the historic center of Puebla and lets out to the Loreto fort, where the Cinco de Mayo battle against the French army occurred. Archeologists first discerned that it was a complex sewer system, but another discovery led them to believe that people also used the tunnels for secret travel.

Along with toys, marbles, and antique kitchen goods, a lot of guns, bullets, and gunpowder were found trapped in the mud. The weaponry was mostly from the mid-19th century, around the time of the Battle of Puebla conflict between Mexico and France. Investigators believe these tunnels may have been used by soldiers during the war of Mexican liberation, though they also could have been used by clergy or even common folk. 

The tour across the tunnel includes a guided visit by archaeologists who worked on the project and displays where one can see the items that lay in the mud for so long. The tunnel museum is known as "Secrets of Puebla," an apt name for the mysterious tunnels that lay hidden beneath the city for so long.