In 1956, in order to accommodate the ever increasing number of pilgrims, a huge underground basilica, nearly the size of Saint Peter's in Rome, was built. The large basilica is entirely underground: for this reason, it is often known as the 'Underground Basilica'. Entering by the main entrance at St. Michael's Gate, the Basilica of Saint Pius X rises on the left side of the Esplanade. This is one of the largest places of worship in the world. Its six entrances facilitates the flow of pilgrims. The main entrance is on the Esplanade opposite The Chapel of St. Joseph. The structure of the interior architecture is made up of 58 small, overturned "V" shaped arches that support the ceiling backbone and the long concrete beam. The centre of the basilica is dominated by a large square altar. The 201-metre long, 81-metre wide basilica has an area of 12000 square metre and an inside perimeter of 475 metres. It can hold about 25000 people.
The Holy Sacrament is kept in the Pax Christi Chapel, also known as the Chapel of the Holy Sacrament, located behind the high altar in the northern wall of the ambulatory. The altar in this chapel contains a relic of Pope Saint Pius X.
The planning and the building of the Saint Pius X Basilica was the work of a group of French architects: Pierre Vago, Andre Le Donnet and Pierre Pinsard. Its design provides maximum visibility making the best use of the space available. It oval shape has been likened to a fish or a grain of wheat, traditional symbols of the Christian Church and of Life. International mass is celebrated here on Wednesday and Sunday mornings. The closing ceremony of the Eucharistic procession and the blessing of the sick also takes place here every afternoon.
Dedicated to Pope Pius X, the Basilica of Saint Pius X was consecrated by Cardinal Angelo Roncalli on 25 March 1958, to celebrate the centenary of the apparitions. A few monts later, he became Pope, taking the name John XXIII. At the far end of the basilica, on the east side, is a third altar, where smaller congregations can gather to hear mass.
Three series of unleaded stained-glass windows, all masterpieces of colour, light the basilica which has no other decoration. The windows are based on original designs by three prize-winners of the Lourdes International Biennial Festival of Sacred Stained-Glass Art. Set along the western ramp are the 15 mysteries of the Rosary by Robert Falucci and the eastern ramp are the 15 Stations of the Cross by Denys de Solere. Completed in 1993, the third series, Bernadette's way of light, is based on sketches by Rene Margotton thus making a total of 52 images. Set around the chevet, it depicts the 18 apparitions and scenes from the saint's life.
The basilica also boasts of three other remarkable stained-glass compositions: Bernadette's Vision by Marguerite Bordet, Our Lady of Light by Tony Agostini and Peter's Boat by Meb.