Only during the construction of the metro network have they discovered what lies beneath Duomo. Before Duomo was built, on this turf there was an Episcopal complex consisting of Basilica Santa Maria Maggiore and Basilica di Santa Tecla, which were demolished in order to build the cathedral. Santa Tecla had an adjacent structure - a baptistery (a separate building used for baptism) called The Baptistery of San Givanni (Battistero di San Giovanni alle Fonti).
This baptistery was discovered just underneeth the entrance to Duomo (around 4 meter underneath the cathedral floor), and it can be visited from the chatedral.
Interesting facts