The Cathedral Basilica of Durango was founded as the Chapel of the Immaculate Conception in the early colonial period. It became the cathedral in 1621, but the building burned shortly thereafter in 1634. It was rebuilt in the middle of the 18th century at the height of the city's mining prosperity, but only a quarter of the construction from that period remains. The church remains dedicated to the Immaculate Conception, which is depicted on the main altar. It also contains Byzantine vaults and a cupola with paintings of angels. Underneath the cathedral runs the Mining Tunnel, also known as the Cathedral Tunnel. It was a mining shaft which also doubled as a secret passage. Today it is a museum.