Vatican Grotto
Vatican City, Vatican City
The Vatican Grottoes are a series of underground chambers and chapels located under part of the nave of St. Peter's Basilica in the Vatican. They are situated three meters below the current floor, and extend from the high altar (the papal altar) to about halfway down the aisle, forming a true underground church that occupies the space between the current floor of the Basilica and that of the old Constantinian basilica of the 4th century. The Vatican Grottoes are not to be confused with the Vatican Necropolis, which is an ancient burial ground that dates back to the Roman times and lies at a lower level beneath the Grottoes and the Basilica.
Archaeological site data from Wikidata (CC0). Summary from Wikipedia (CC BY-SA).