The church of Santa Maria in Cosmedin was built in the 6th century on the ruins of the Altar of Hercules, which are still visible from the crypt. It was one of the most important churches of the Roman deaconate, a kind of parish church which, at a time when the state institutions were inefficient, ministered to the material as well as the spiritual needs of the faithful.

Today's church still uses the Byzantine rite for the Mass. However, the church is famous for an absolutely profane element, the Bocca della Verita' in the atrium. Placed here in 1632, the Mouth of Truth is really a manhole cover from the classical age that takes the form of a river god's mask. Legend has it that the sharpness of the aperture would mutilate any liar who put his hand inside.
Opposite the church, there are 2 well-preserved temples that date back to the 2nd century AD. The rectangular one was dedicated to Portunus, the god who protected the river port of Rome, whereas the circular temple is dedicated to Hercules Olivarius. It was constructed to honor this god by a Roman merchant who got rich selling olives.