They offer a Soterranea experience, but in small scale, compared with the other one with the cisterns. They insist that this tour is non calustrophobic and that it absolutley true, all wide spaces. The other one is impossible for claustrophobics.
In the underground part, you’ll see a market and a private house. Then, with the same ticket, you can visit the basillica and the beautiful Chapter hall in the cloister.