[3]: 7  The site chosen for the baths formerly belonged to a vast garden estate known as the horti Asiniani, developed by Gaius Asinius Pollio during the reign of Augustus.

[4] The Farnese Bull sculptural group that was later moved into the Baths of Caracalla was already present at the site in the time of Pollio, who had imported it to display in his gardens.

[4] The estate of Pollio was appropriated by Caracalla to build his baths; the existing structures were demolished in some cases to their ground floors, filled in with earth and incorporated into the foundations of the new complex.

[5] Recently this two-storey domus deep below the baths, built in 134-138 during the reign of Hadrian,[6] and partially demolished in 206 has been opened to the public revealing lavishly decorated frescoed ceilings and a lararium, a room dedicated to Roman and Egyptian gods.

[7] For work to have been mostly completed in the time of Caracalla, workers would have to have installed over 2,000 tonnes (2,200 short tons) of material every day for six years.

In the mid-4th century a Christian pilgrimage site came into being, known as the titulus Fasciolae, close to the baths near the Church of Santi Nereo e Achilleo.

[4] In 537 during the Gothic War, King Vitiges of the Ostrogoths laid siege to Rome and severed the city's water supply.

[9] Located too far away from the still-populated area of Rome, the baths were mostly disused but in the 6th and 7th centuries were apparently used for the burials of pilgrims who died after being cared for in the nearby xenodochium of Santi Nereo e Achilleo.

[10] At least since the 12th century the baths were used as a quarry for construction materials, and of decorative pieces to be reused in churches and palaces (e.g. in Pisa Cathedral and Santa Maria in Trastevere).

In the 15th century, Pope Pius II used stone from the Baths in the construction of the Loggia of the Benediction at St. Peter's Basilica.

[13] The excavations of Paul III between 1545 and 1547 uncovered many large statues made of marble and bronze, as well as innumerable architectural fragments, lamps, intaglios, and cameos.

Philip Neri may have brought children from his oratory here—he is believed to have commissioned the fresco Madonna supported by an angel still located in the natatio.

[3]: 10–2 Between the 16th and 18th centuries interest in the structure was rekindled and several famous architects made drawings of the ruins (Andrea Palladio, Giovanni Battista Falda, Giambattista Nolli and Giuliano da Sangallo).

[citation needed] In 1824, excavations at the baths were conducted by Count Egidio di Velo, whose findings included the mosaics showing athletes now at the Vatican Museums.

[3]: 12 From 1866 to 1869 restoration work in the central part of the complex revealed a torso of Hercules, porphyry columns and figure-adorned capitals.

[3]: 14 From the early 20th century, excavations expanded into the outer areas of the complex and downward, revealing the subterranean passages, including a Mithraeum (see below).

The complex is rectangular, measuring 337 m × 328 m. Its construction involved the moving of a substantial amount of earth, as parts of the nearby hills had to be removed or leveled into platforms.

The main building stood in the centre with no connections to the surrounding walls, which housed the cisterns, two symmetrical libraries (south), two large exedras (east and west) and tabernae (shops) to the north.

However, dressing rooms and palaestra were arranged symmetrically on both sides of the building, giving easier access and facilitating the flow of people in and out of the thermal area, thus increasing the number of potential users of the baths.

[3]: 22 The bath main building was 214 m × 110 m and the height to the top of the roof line was 44 m (145 ft); it covered 2.6 hectares (6+1⁄2 acres) and could hold an estimated 1,600 bathers.

The hall served a dual purpose: It was a meeting place and transition area for visitors heading for other parts of the bath.

[3]: 32–3 The Olympic-sized natatio pool measured 50 m × 22 m. The walls rose to more than 20 m and the northern façade was structured by three huge columns made from grey granite.

[17] Discovered in 1912 by Ettore Ghislanzoni, the Mithraeum at the baths is considered the largest documented gathering space for the worshippers of Mithra, the Persian god in vogue with the military and mostly lower-class men, in the 2nd and 3rd centuries AD.

In the floor near the entrance a circular pit was found covered with a marble slab, with a terracotta bowl containing the remains of staves of wheat.

A rectangular hole in the floor gives access to a small tunnel that runs under the center of the main hall into an adjoining room, where there is an entrance/exit with staircase.

Most scholars view this as a fossa sanguinis, a ritual pit over which the bull at the center of the Mithraic mythology was slaughtered, bathing one or more initiates in its blood.

Although many were destroyed in the Middle Ages to make lime, beginning in the 16th century under Pope Paul III Farnese, sculptures were excavated from the area to serve as decorations in newly built palazzi.

[3]: 51  In the Lateran Collection of the Vatican Museums are mosaics composed of rectangular panels depicting athletes, which were discovered in 1824 in two of the library exedrae to the east and west of the baths complex.

[3]: 32 In the 19th and early 20th century, the design of the baths was used as the inspiration for several modern structures, including St George's Hall in Liverpool, the original Pennsylvania Station (demolished in 1963) in New York City, the current Senate of Canada Building in Ottawa and the main hall of the University of Leipzig (demolished in 1968).

In 2001, use of the venue by the opera resumed but it now uses a temporary movable stage outside of the main structure, which reduces the stress on the antique ruins.