A tiny church located near the banks of the Tiber on the beautiful Via Guilia and quite near Piazza Farnese, is one of Rome's many little secrets. It predates the better known Capuchin Cemetery by almost sixty years, yet contains similarly ghoulish decorative motifs. Its name is translated as "Saint Mary of Prayer and Death", and was built for a confraternity of monks responsible for interring corpses found on the streets (and surely the Tiber) of Rome. It is only open for very limited hours every Sunday. On our first visit, my wife and I found a simply decorated interior (the facade is where expectations rise, as you're greeted by various sculptural skeletons, etc.) and a flight of stone stairs. We descended to find ourselves surrounded by shelves of skulls in various conditions and a large cross made entirely of human skulls. Unlike the Capuchin Cemetery on the Via Veneto, this church always seems without tourists, and as such has my highest recommendation. There may be little instructional value here (no "Ars Moriendi" or "art of dying"), but as a simple encounter with one's own mortality, it can't be beat.