I lived near Arles for a year before I discovered the cryptoporticos. And with good reason: they are several meters underground at the city’s center, Place de la Republic and Place du Forum.
Cryptoporticos, typical of early Roman architecture, were built underground to support porticos, hence the name. The cryptoporticos in Arles supported the Roman Forum, and are one of several important such sites in Europe. The Roman Forum at Arles was one of the most important Roman centers outside of Italy.
Probably built in 30-20 B.C., and most certainly in the 1st Century B.C., the Roman Forum at Arles was a center of economic, religious, and judicial life for the Romans, who conquered Arles in 123 B.C. Underground passages with domed ceilings supported the traditional porticos that surrounded the Roman forums, giving them a slight elevation to signify their importance. In 46 B.C. Julius Ceasar founded a Roman colony in Arles. The cryptoporticos at Arles, built soon after, were constructed in a U-shape, and consist of three parallel passageways. Stonemason marks indicate that the underground foundation was probably built by Greek masons living in Marseille.
The cryptoporticos at Arles were ingeniously built into a natural slope in the terrain. The southern portion of the foundation would have been underground and the northern portion would have been at ground level. It has been suggested that these could have been used as shops at some point; however, some believe that civil slaves were kept here during it’s earliest use.
Today, all of the structure is underground, and the Roman Forum has been replaced by the town’s municipal building and the Chapel of the Jesuit College. Access to the underground site is rather obscure and gained through a small office located inside the municipal building. After descending a stairway, you take a step back many centuries to walk the dark passageways in the horseshoe-shaped structure, completely underground and in a remarkably well-preserved state.