Start your tour at the front of St. Peter's Basilica and go to the right of the front entrance by the elevator to the cupola. No admission is charged, but there might be quite a line, especially at the height of tourist season, on Wednesdays (when the Pope gives public audiences), or if there's a special event, like the canonization of a saint. Entrance to the grotto used to be by the main altar, but the post-John Paul crowds caused Vatican officials to change the route. The grottoes are open from 7:00 a.m. to 6:00 p.m. April through September and 7:00 a.m. to 5:00 p.m. October through March.
Descend to the level of the grottoes and pass the Gallery of Clement VIII, the Chapel of Our Lady, Queen of the Hungarians and the Mosaic of John VII. Turn right, and you'll see the simple tomb of Paul VI, with its slanted stone marker. Paul VI reigned from 1963 to 1978, presiding over much of the Second Vatican Council and supervising the implementation of the changes it authorized.
Walk on and you'll see the tombs of Marcellus II on the right and John Paul I on the left. John Paul I reigned for slightly over a month in 1978, just long enough for the world to be captivated by his smile and unpretentious manner. His tomb is made of gray streaked marble and is flanked by statues of angels. Beyond this are the tombs of Innocent IX on the right and Benedict XV on the left. Benedict reigned from 1914 to 1922 and much of his tenure was spent trying to bring a peaceful end to World War I. The marble tomb is topped with a bronze effigy of the pope. Next up are the tombs of Queens Christina of Sweden and Charlotte of Cyprus, to the right and left, respectively.
Step forward and on the right is the simple tomb of John Paul II, who reigned from 1978 to 2005. Like Paul VI, he was interred in the floor, with a slanted stone marker. Originally the tomb of John XXIII was located in this semi-circular room, but his remains were later moved to a glass tomb upstairs. John Paul's tomb evokes strong emotions in visitors and some linger and pray for quite a while in an area opposite the tomb. Guards will be happy to place flowers in the room or briefly place rosaries or other sacramentals on top of the tomb for the purpose of garnering a blessing. A bronze grille on the main floor of the Basilica looks down into the room and a live webcam keeps watch around the clock.
Pass the Chapel of the Madonna and the tomb of Pius VI and turn into the second opening on the right--the first section of the curving Peribolos corridor. This will take you past a series of chapels to the far west end of the grottoes and the unornamented white tomb of Pius XII, who reigned from 1939 to 1958. Pius was pope during World War II and due to his strong opposition to communism, the Soviets subjected him to a posthumous smear campaign, in which despite evidence to the contrary, he was depicted as an anti-Semite and supporter of Hitler.
Turn around and proceed east down the corridor and you'll wind up in the elaborately gilded Clementine Chapel. Behind the altar and a bronze grille rest the remains of St. Peter, the rock upon which Christ vowed to build His Church. Next either retrace your steps or turn right out the side exit, then right twice more. You'll pass several more chapels, including, on the left, one dedicated to Our Lady of Guadalupe, the Patroness of the Americas. Further on to the left is the Central Altar of the grotto, while in the niche to the right is the tomb of Pius XI, who reigned from 1922 to 1939. A delicate marble effigy of Pius tops the tomb and the walls of the niche are lined with mosaics. Beyond this, on the right, are the tombs of Cardinal Merry del Val, James, Charles and Henry (the last Stuart claimants for the British throne) and Popes Innocent XIII and Urban VI.
At the eastern end of the grottoes are the tombs of Popes Hadrian IV and Gregory V, Emperor Otto II and Popes Julius III, Innocent VII, Nicholas III, Boniface VIII and Nicholas V. Turn left at the early Christian Sarcophagus. Go through the three archaeological rooms (there are three more alongside them to the west) and exit the grottoes at the Piazzetta Braschi.
