The site was chosen for more than its elevation. Montmartre, the "Hill of the Martyrs," is where tradition holds that Saint Denis, patron saint of Paris, was beheaded by the Romans. It was also where Ignatius of Loyola and his followers took their vows in 1534, founding the Society of Jesus. When the National Assembly approved the basilica in July 1873, the official statement declared it necessary "to efface by this work of expiation the crimes which have crowned our sorrows." The architect Paul Abadie won a competition that drew seventy-seven proposals, and the cornerstone was laid on June 16, 1875. But the ground beneath was treacherous. Eighty-three wells, each thirty meters deep, had to be dug and filled with rock and concrete to create underground pillars. The estimated seven million francs in construction costs, drawn entirely from private donors, were spent before anything rose above ground level.

Construction took four decades and outlasted five architects. Abadie died in 1884 with only the foundations complete. His successors made extensive modifications, but the real obstacles were political. When Georges Clemenceau's left-wing coalition won the parliamentary elections in 1882, they immediately tried to halt the project and blocked all government funding. Only the threat of twelve million francs in cancellation liabilities forced them to let it continue. Opponents then proposed an extraordinary countermeasure: erecting a full-size replica of the Statue of Liberty on Montmartre, directly in front of the basilica, to block it from view entirely. The plan was abandoned as too expensive. A bomb was detonated inside the church in 1976. In 2004, the socialist mayor renamed the square below in honor of Louise Michel, the anarchist Communard. Former Prime Minister Lionel Jospin publicly wished the building demolished.

Sacre-Coeur's gleaming whiteness is not an illusion, and it is not maintained by cleaning crews. The building is clad in travertine limestone from Chateau-Landon, quarried at Souppes-sur-Loing in Seine-et-Marne. This stone has a remarkable property: on contact with rainwater, it exudes calcite, effectively bleaching itself. While most Parisian buildings darken with age, Sacre-Coeur grows whiter. The Romano-Byzantine design was itself a deliberate provocation, rejecting the neo-Baroque style of Charles Garnier's recently completed Opera nearby. The immense central dome rises 83 meters, surrounded by four smaller cupolas. At the north end, an 84-meter bell tower houses the Savoyarde, the largest bell in France at nearly nineteen tons. Cast in Annecy in 1895 and hauled to Montmartre by twenty-eight horses, it commemorates the attachment of Savoy to France. Its sound carries ten kilometers.

Inside, the basilica is deliberately dim. The windows sit so high above the altar that sunlight enters obliquely, creating a mystical atmosphere quite unlike the bright Gothic interiors elsewhere in Paris. The apse mosaic, one of the largest in the world, depicts Christ with arms outstretched, flanked by scenes titled "The Homage of France to the Sacred Heart" and "The Homage of the Catholic Church to the Sacred Heart." The Latin inscription at its base reads: "To the Sacred Heart of Jesus, France fervent, penitent and grateful." The word "grateful" was added after World War I. Below this mosaic, since 1885, a monstrance displaying the Blessed Sacrament has been maintained in perpetual adoration, an unbroken vigil that began before the building was even finished. Today, Sacre-Coeur draws more visitors than any Paris landmark except the Eiffel Tower, its dome offering views across the entire city from two hundred meters above the Seine.