Reopened to the public in 2015 after major restoration work, the Crypt of the Sanctuary is the heart of the history of Santa Maria del Monte. Inside are the remains of the first church built in the 9th-10th centuries, as well as important archaeological evidence attesting to the antiquity of the cult. In 2013, in fact, excavation work in the small Romanesque crypt with three vaulted naves uncovered some significant wall and floor fragments that clearly document the pre-existence of a much older building of Marian worship, to date unknown and datable to the 5th-6th centuries.
The frescoes decorating the crypt are also of particular interest. They present homogenous stylistic features, which can be traced back to the same craftsmen who intervened at different times, not far apart. Scholars disagree on their dating: according to some, the various scenes were executed between 1360 and 1370; according to others, their execution should be advanced to the early 15th century. Undoubtedly, they are the work of a local workshop that successfully repeated images with a rigid frontality, but perfectly recognisable, making the devotional aspect prevail over the artistic one.
The crypt is reached through two corridors, where important discoveries were made during the restoration works: in the first corridor, devotional frescoes dating back to the 15th century and of a high quality; in the second corridor, the remains of a funerary oratory with burials dating back to the Carolingian period and with older phases dating back to the Longobard period.