The curving brick facade on the northeast side of Piazza della Repubblica is one small remaining part of the colossal Terme di Diocleziano, the largest and most impressive of the baths of ancient Rome. A gift to the city from Emperor Diocletian, it was erected about AD 300 with the forced labor of 40,000 Christians. In 1561 Michelangelo was commissioned to convert the vast tepidarium, the central hall of the baths, into a church. His work was altered by Vanvitelli
in the 18th century, but the huge transept, which formed the nave in Michelangelo's plan, has remained as he adapted it. The eight enormous monolithic columns of red granite that support the great beams are the original columns of the tepidarium, 45 feet high and more than 5 feet in diameter. The great hall is 92 feet high.