Santa Maria Sotto L'Arco was a little chapel dedicated to the Blessed Virgin Mary in the Vatican, on a site now occupied by the Pope Paul VI Audience Hall.
The Nolli Map of 1748 shows the Vicolo del Leoncino running south of the south-east corner of the church of Santa Maria della Pietà in Camposanto and passing under an archway, the Arco del Leoncino, to reach the courtyard of the Abitazione dei Cavalligieri. The chapel entrance was under the archway.
The entire street now lies under the Aula Paolo VI, which was completed in 1971. The chapel was in what is now its south-east corner.
It may be noted that the aula is actually outside the boundary of Vatican City, being an extraterritorial possession of hte Holy See in Italy.