Lourdes, perhaps the most significant site of pilgrimage in the world for Catholics, serenely snuggles at the foot of the Pyrenees Mountains in south-western France. The religious heart of the town aptly named, Cité Religieuse, is embraced by the Gave de Pau river, which meanders through this special township.

Bridges across the river lead to a spacious park, the Esplanade des Procession, where hundreds of faithful in hushed voices, some being pushed in wheelchairs, make their way along tree-lined trails crossing the green expanse to Basilique du Rosaire et du L'Immaculé Conception ahead. The Basilica with its arching extensions seems to welcome the pilgrims entering this mosaic-decorated, holy place.

After a reverential pause, the journey continues to the crypt covered in offerings thanking the Madonna for a grace received. Ascending to the upper Basilica, marble statuary, stained glass windows narrate biblical stories and exalt the power and goodness of God. To the right of the Basilica, by the river, is the entrance to a moisture-blackened grotto, guarded by a tall, life-size statue of the Virgin Mary looking down from an oval niche.

Pilgrims, sometimes in their thousands, crowd before the grotto and meditatively gaze at the famed statue. At the deep end of the grotto, a spring gushes out water which some Catholics believe has miraculous properties.

Lourdes owes its fame to a singular event that took place 150 years ago in this very riverside cave, Grotto de Massabielle. On 11 February 1858, a 14-year-old peasant girl, Bernadette Soubirous, had a vision of the Madonna, who pointed to the spring in the grotto and said, “I am the Immaculate Conception, go drink at the spring and bathe in its waters.”

By 16 July, the same year, Bernadette had had 18 such visions. She spread the word about these Marian apparitions and the healing powers of the spring. What began as a trickle of pilgrims to the sleepy village grew and grew. The organisation of pilgrimages was then taken over from the local priest by a Catholic movement called Assomptionistes, who brought the first large-scale pilgrimage to Lourdes in 1873.

As the trickle of pilgrims became a river by 1873, the Basilique du Rosaire was already being built to accommodate the masses. By the turn of the 20th century, the Basilica was opened but it was soon realised it wouldn’t hold the increasing flow of people coming to the holy town. The foundations of a new subterranean basilica, St Pius X, were laid nearby; it was planned to hold up to 20,000 and was finally consecrated in 1958, the first centennial of Bernadette’s visions, by Cardinal Roncalli, the future Pope John XXIII.

As the focus of pilgrimage is really Grotte de Massabielle, the site of the apparitions, further space was needed, hence a semi-circular area, Espace St Bernadette, including a church, was created just across the river in 1988 to hold 7,000 more faithful.

In the 1960s some two million pilgrims came here; today over six million visitors come to Lourdes annually, making it the most popular destination of Catholic pilgrimage in the world. Lourdes is now a growing town counting 20,000 or so permanent inhabitants, most of whom are providing a service to the pilgrims in one way or another. There are more hotels and souvenir shops here than in any other place in France except Paris.

But why do so many come to Lourdes and what exactly do they experience? I quiz two Italian pilgrims of two very different generations. A thirty-something secretary says, “I’d heard so much about Lourdes, I wanted to see, understand what it felt like to be here. Walking into the Basilica Rosaire, I felt an extreme sense of peace, I’ve been in many churches before, but this was different, the feeling of peace was much more profound.”

The 78-year-old retired shop assistant confirms much the same and adds, “Looking at the statue of the Madonna, seeing all these people, you feel you belong to a community. It confirms my belief in our religion. I also wanted to thank Our Lady of Lourdes for saving my life. When I was 12 I had peritonitis, and nearly died. My aunt rubbed holy water from Loudes on my tummy, the pain eased, then I had a risky operation and everything went fine.”

Not everybody comes to Lourdes just to show devotion to Our Lady, many also ask for help, drink the water from the spring in the grotto, or bathe in its waters in special tubs and pray for a miracle, a cure to a serious or otherwise intractable ailment conventional medicine hasn’t been able to treat. Others still come to offer thanks.

An Argentinean couple tells me, “We’ve come to thank the Virgin Mary for her blessing. Our son was born brain-damaged; he can’t walk, but it’s only a physical impediment. He’s a bright 8-year-old now, and doctors say that in time, with continued rehabilitation, he’ll be able to walk, imperfectly, but good enough to have an independent life. We couldn’t have asked for more.”

Unlike what most people probably think, only a fraction of the six million plus pilgrims come to Lourdes in search of a miracle from Mary. There is a thorough and long-drawn out process of investigation of all claimed cures. Over the last 150 years the Catholic Church has recognised just 67 such healings, of which only 10 have been recorded in the last 50 years.

The first, officially recognised, otherwise unexplainable, miraculous cure was enjoyed by a 38-year-old French woman, Catherine Latapie, whose paralysed forearm began functioning again after her visit to Lourdes. The most recent occurred to a French gentleman aged 51 who recovered from multiple sclerosis in 1987, but his case was only recorded as miraculous in 1999.

The 67th miracle, granted to an Italian woman, Anna Santaniello, made the official records in September 2005 although the healing took place back in 1952. Doctors who examined the heart of the Italian lady, now 93, realised she had long recovered from the dysfunctioning mitral valve in her heart after a visit to Lourdes 53 years earlier.

Bernadette in 1866, aged 22, joined the Sisters of Charity as a novice at Nevers taking annual vows to do charitable and religious work. She became a nun in 1878, a year before she died in a convent. She was beatified in 1925 and canonised in 1933 becoming St Bernadette.

Her legacy lives on in Lourdes, where there is a museum to her at Pavilion Notre-Dame. Her birthplace at Moulin de Boly, her father’s house at Cachot and the family house at Centre Hospitalier at the time of the Marian apparitions are all places carefully looked after to preserve her memory. The 11 February is the official feastday of Our Lady of Lourdes. A small grotto by the river is now a major shrine where millions come to seek peace, comfort, reinforcement of their faith and perhaps a miracle.