One of the peculiarities of this church is that it was obtained from the Diocletian thermal baths, the biggest ones ever built by the romans. They were where today is Piazza della Repubblica (below the square in the metro station is still possible to see few ruins) and the church has been built using part of the thermal baths. The facade itself seems to be a ruin. The project of utilising ancient ruins for a church is from Michelangelo. Vanvitelli in 1750 placed inside the church many paintings coming from St. Peter. They were eventually replaced by mosaics (you can notice walking inside St. Peter that there are no more paintings on the walls, but just mosaics). Worth to note is the meridian in the transept. From 1700 to 1846 it was used to scientifically demonstrate the correctness of the gregorian calendar and to determine the exact day of Easter according to the rotary motion of the sun and the moon.