Among the many holy places and religious destinations Holly Hayes has visited, the spired monument in Cologne, Germany [Images], is among her favorites. "To stand in front of the astonishing, soaring Gothic architecture at Cologne Cathedral is to begin to understand majesty," she says.

Hayes, a self-described "religious historian who loves to travel," is the founder and editor of the website Sacred Destinations, and while she says her interest in holy places is "more cultural and architectural than spiritual," she felt something special at Cologne.

"It was Christmas Mass there," she recalls, "and the huge nave was packed full of worshippers bundled up in their winter coats. Watching the solemn processions and listening to the Latin readings in the magnificent medieval surroundings, I felt transported back in time, as if I were participating in a small way in a very long, unbroken tradition of faith and history." Approximately six million visitors come to the Cologne Cathedral each year. Some come to the church out of historical or cultural interest, others out of religious devotion. Still others to take a picture of a place they read about in a guide book.

We've gathered visitation figures for 20 of the world's top religious destinations, and while it's clear that the world's sacred sites draw millions upon millions of visitors, travel to shrines, churches, temples, mosques or other holy place can come from a mix of motivations.

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"On one end of the spectrum you have religious devotees who go to have a spiritual experience; and on the other you have the curious who want to see a Buddhist temple, for example, and are interested just from a cultural or historical perspective," says Dallen Timothy, professor in the department of geography at Brigham Young University and author of Tourism, Religion and Spiritual Journeys.

"A lot of religious organizations, and countries like Saudi Arabia and Israel, with huge groups of pilgrims, refuse to call them tourists," he says. "But the distinction between tourist and pilgrim is a false dichotomy," he argues. "Religious travel is one of the biggest forms of tourism in the world."

The ForbesTraveler list seems to confirm this: Every year, massive numbers of travelers visit the sites of saints' and prophets' tombs, or to partake in annual celebrations at holy shrines. At the Our Lady of Guadalupe Shrine, in Mexico City, where the Virgin Mary is believed to have appeared in the 16th century to a poor peasant, estimates for annual visitation to the basilica run as high as 20 million. According to a 1999 Vatican Council report, it was the most-visited Catholic shrine in the world.

At Sabarimala, a Hindu pilgrimage center in South India, visitation estimates vary from five million to as high as 60 million annually. Juan Campo, a professor in the religious studies department at the University of California Santa Barbara, was recently in Sabarimala during pilgrimage season, and guesses that the actual number might be closer to 10 million. "It's a large shrine located in the mountain area," he says, "and people come there primarily from South India. But you also have South Indians who work abroad, and they'll return home to participate in the pilgrimage. It's a constant influx of people."

Campo also studies pilgrimages to Guadalupe and Islam's holiest site, Mecca, in Saudi Arabia. No matter the differing faiths, nearly all pilgrims "will say that at some point (they had) an experience of intimate contact with the divine -- some sense of a sacred experience that they remember very clearly."

Hayes agrees. "Regardless of their position or form (there) is a direct connection with the holy, the spiritual, and the supernaturally powerful. Pilgrims seek places where gods or saintly humans have walked and miracles have occurred, in the hopes they might participate in it and/or benefit from it themselves."

In Lourdes, France [Images], where in 1858 a 14-year-old girl reported multiple visions of the Virgin Mary, that benefit comes in part from spring water that is said to possess healing powers. An estimated five million pilgrims a year come to follow the injunction of the message written above the spring: "Wash your face, drink this water and pray God to purify your heart."

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Suzanne Kaufman, professor of history at Loyola University of Chicago and author of Consuming Visions: Mass Culture and the Lourdes Shrine, says, "When one visits Lourdes today, one is confronted with two remarkable sites: crowds of desperately sick pilgrims drinking, bathing and praying at the Catholic shrine, and equally large numbers of customers shopping at hundreds of piety shops that line the major boulevards leading to the pilgrimage site."

Kaufman says the mass-marketing of Lourdes was in full force even a hundred years ago, when "the Augustinian Fathers of the Assumption, a Paris-based religious order, made Lourdes the site of their national pilgrimage. With the help of the railway and the Catholic popular press, the Assumptionists transformed Lourdes into a site of mass pilgrimage, bringing hundreds of thousands of devout Catholics to the shrine."

The ease and luxury of transportation, of course, is part of what distinguishes the modern pilgrim from faithful journeyers in the past, who may have trekked long distances on foot to pay homage to their god. "The word 'travel' comes from ancient word 'travail,' which means hardship," explains Dallen Timothy. "It used to be that the trail or pathway was more important than the destination. The original notion of pilgrimage, regardless of religious tradition, was that getting there was more important than the destination."

Today, Muslims can book five-star Haj packages and Christians can join luxury cruises in Greece to follow in the footsteps of St Paul. Religious destinations and pilgrimages have become more commodified. Says Timothy, "Every religion does it."

But while the journey aspect of pilgrimage may be greatly diminished, Holly Hayes says, "I'm not sure this means the essence of pilgrimage has been lost in modern times. If a medieval pilgrim were offered a trip to their destination on an airplane -- even in economy class -- I find it hard to believe many of them would refuse. That's because, I would argue, the most important thing is to visit the shrine, to be close to the holy and the supernatural."

For a closer look at the numbers of faithful -- and curious -- and where they flock, take a journey through our slide show. As the wildly divergent estimates for Sabarimala attest, data on visitation to religious destinations can be less than reliable, as can the definition of "religious destination" -- which, in some broad definitions includes places like Civil War battlefields or religious gatherings. The periodic Hindu convergence, Kumbh Mela, for example, rotates among four locations.

With an estimated 70 million in attendance, the most recent Kumbh Mela would top our list, but we've chosen a more conventional definition: looking at physical sites that are part of present-day religions (which excludes heavily visited sites such as the Egyptian Pyramids or the Parthenon in Athens). We've gathered figures from tourist boards, the sites themselves, scholars, reputable media sources and, where possible, a combination of these.

Although they are listed in approximate order, according to the best sources available, we are not giving religious destinations hard-and-fast number-rankings. Other sources for these estimates may surface, which we will monitor and -- if persuasive -- will adjust accordingly.

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Photograph: Crowds gather in January at the Ardh Kumbh Mela held in Allahabad. Photograph: Mario Tama/Getty Images