The Baths of Diocleziano were the largest among all the spas built in Rome and the Roman world. Built in less than eight years, between 298 and 306 AD, the Baths, dedicated to Diocleziano, was started by Massimiano on his return from Africa and endend after the abdication of Emperor Domiziano, and Massimiano. Many buildings were demolished to make way for a gigantic building that occupied an area where a great basilica was built in the central complex caldarium - tepidarium - natatio positioned along the minor axis, gyms on the sides of the major axis.

On the north-east was the great tank trapezoidal called “Tub of Termini”, whose last remains were demolished in 1876. Here came the branch of the Marcia water that supplied the baths and whose name remains, deformed, in the place name Termini, which is passed to designate the train station. To imagine the size, just think that more than 3000 people were able to simultaneously use the services of the bath. On the north-east side of the square, next to the Faculty of Education, you can still see the remains of one of the apses, which opened in the walls of the caldarium, now largely disappeared.

Another of them, well preserved, is now the entrance to the church of Santa Maria degli Angeli, which occupies the central area (called “basilica”) of the bath. The exedra station, which served as the auditorium to watch the gymnastics exercises is repeated, and its development by the monumental colonnade curving buildings architect Koch located on either side of this square once named, precisely, Exedra. The western corner of the Baths constited of a large octagonal room with a semicircular niches on the four corners, made from the beautiful environment of the Planetarium. The church of Santa Maria degli Angeli was obtained by Michelangelo in 1563-66, from Tepidarium room, the baths of warm water, which now forms the vestibule of the church itself. Impressive are the eight columns of red granite, skylights, belonging to the Baths, hidden in a stretch of the lower part, having had to raise Michelangelo six feet on the floor. Was remodeled by Vanvitelli in 1749. At the center of the square you can admire the beautiful Fountain of the Naiadi, which is the exhibition of the Marcia water, put forward by Quinto Marcio Re in 144 BC at the high valley of Aniene near Arsoli.

The story of this fountain begins in the second half of last century, when pope Pio IX gave appropriate provisions for the reconstruction of the ancient aqueduct Marcio: the water, in honor of the pope, was called Acqua Pia. The first exhibition, consisting of a small circular tank at ground level, topped by flickering of jets, the central most powerful, was devoid of any architectural or sculptural ornament and was placed in the presence of the pope, then at the square Termini, where is now the memorial Dogali, 10 September 1870. The events of that time, led the people who coined the slogan: “Acqua Piar, now yours, mine tomorrow”: ten days later, in fact, with the capture of Rome, fell to the temporal power of popes.

In 1855, with the approval of the master plan and the start of major renovations city, was decided on the arrangement of the large square of exedra and it was decided that the definitive exhibition of the water Marcia arising in square itself, background of the axis of via Nazionale. In 1888, whitin the designated area, was built the current fountain, designed by the engineer A. Guerrieri: four small lions “of plaster” were placed on the fountain, on the occasion of the visit to Rome of the Emperor of Germany.