You could easily walk through Piazza di Porta S. Giovanni and overlook an elegant, eggshell-pink and marble edifice in the shadow of the Lateran Palace. What looks like a modest church is in fact one of the most important religious sites in the world, visited by more than two million people a year. A third phase of restoration is now under way, which will bring its faded interiors and precious frescos back to their original luminous splendour.

The sanctuary houses two important structures