Basilique du Sacré-Cœur de Montmartre

The brilliant white facade you see crowning the hill is composed of travertine stone from the Chateau-Landon quarry. This specific stone is unique because it constantly exudes a substance called calcite when it comes into contact with rainwater. This chemical reaction acts as a natural bleach, ensuring that the basilica retains its radiant, pearly appearance against the Parisian sky even after more than a century of exposure to the elements.