San Carlo alle Quattro Fontane (San Carlino)

Francesco Borromini's first independent architectural commission (1638-46) and a foundational masterpiece of Baroque architecture. The entire footprint is so small that it could fit inside a single supporting pier of St. Peter's — yet within that constraint Borromini created an undulating facade of concave and convex curves, an oval coffered dome of geometric precision that seems to hover unsupported, and an interior of constantly shifting wall planes. Romans nicknamed it 'San Carlino' (Little San Carlo) for its size. Borromini died by suicide in 1667 at age 67; he was buried elsewhere. The small adjoining cloister and crypt are accessible for an extra fee.