The Basilica of St. Pius X lies entirely underground, accessed by a ramp that descends from the Esplanade des Processions. The scale inside is remarkable — a vast, low concrete vault held up by twenty-two elliptical arches, with a capacity of 25,000 people. The building was designed by Pier Luigi Nervi, one of the great engineers of the 20th century, and the structural elegance of the concrete arches is impressive despite the utilitarian purpose.
The basilica is the venue for the largest Mass gatherings in Lourdes, particularly on major feast days when hundreds of thousands of pilgrims converge on the sanctuary. It operates as a simple, functional space most of the time — the interior is deliberately plain, with the focus on the congregation rather than decoration. Seeing it empty or partially occupied gives a better sense of the engineering; when it is full, as it can be during major pilgrimages, the experience is something else entirely.