Vatican Grotto, Underground mausoleum in St. Peter's Basilica, Vatican City.

The Vatican Grottoes lie three meters below the floor level of St. Peter's Basilica and form a network of chambers, chapels, and burial vaults connected by several corridors. This underground area extends directly beneath the nave and houses numerous tombs alongside archaeological remains from earlier structures.

The grottoes occupy the gap between the floor of the present St. Peter's and that of the 4th-century Constantinian basilica, which stood until the 16th century; this is why they run only as far as the old nave did. The area is laid out as a three-aisled church with chapels for papal tombs, the so-called old grottoes, and a semicircular apse, the new grottoes, centred on the Chapel of St. Peter beneath the papal altar and Michelangelo's dome. Excavations for St. Peter's tomb began in 1940 and were kept secret for ten years, even during the Second World War; Pius XII announced the find by radio in the Holy Year of 1950.

The grottoes hold the tombs of 21 popes, from the apostle Peter to Benedict XVI, whose remains lie in the niche that earlier held John XXIII and John Paul II. Not only pontiffs are buried here: Queen Christina of Sweden, Queen Charlotte I of Cyprus, Cardinal Rafael Merry del Val and the Stuart prince James Francis Edward with his two sons also rest in these chambers.

Access is through the basilica, and visitors follow marked pathways through the underground corridors. The low ceiling height and narrow passages require walking slowly, and some areas have limited accessibility for wheelchair users.

The central area holds the Clementine Chapel with a 17th-century altar positioned near St. Peter's tomb, marked by white marble covering. This spot lies lower than the surrounding burial chambers and marks the spiritual heart of the site.