In the vicinity of Via Monastero Maggiore, at number 2 is the Celtic Hypogeum. A place with many underground chambers, dug into the rock using primordial techniques, and branching off on several levels in an approximate K-shape. A steep staircase leads to the central chamber, from which three corridors branch off. There are niches and unfinished pallets in the walls, but most intriguing are three masks. The original function of this peculiar monument, which has no parallel in Friuli, is still a mystery. With the exception of fanciful interpretations mixed with legend, a funerary use of the hypogeum in Celtic times has been hypothesised, while some scholars believe that the rooms were prisons in Roman or Longobard times. The hypogeum appears to have been carved out of a natural cavity located to the right of the Natisone River, which flows below. Given that the natural opening to the river was then closed, it may have functioned as a water cistern. Connecting steps were carved into the rock, as well as stone seats and benches, pillars and two roughly carved fascinating faces that look like ancient cult keepers. The presence of water, which wets the floor surfaces and seeps through the porous rocks, may be an indication of the rituals unknown to us. On the left, it is possible to see two stone seats, which inevitably narrow the passage: here, perhaps in connection with the ritual, two people could be seated.