Excavations at Ostia Antica have brought to light 26 bath complexes in the town. These range from large public baths, such as the Forum Baths, to smaller, often private ones, such as the small baths (Regio I, XIX, 5).
Baths in Ostia, like elsewhere in the Roman world, would have served both a hygienic and a social function. Bath construction increased after an aqueduct was built to Ostia in the early Julio-Claudian Period.
Many of the baths follow simple linear arrangements, with one room following the next, due to the density of buildings in Ostia. Only a few, like the Forum Baths or the Baths of the Swimmers, had the space to include a palaestra. Archaeologists name the bathhouses from features preserved, for example the inscription of Buticoso in building I, XIV, 8 leads to the name Bath of Buticosus or the mosaic of Neptune in building II, IV, 2 leads to the Baths of Neptune. The baths in Ostia follow the standard numbering convention, which divides the town into five regions, numbered I to V, and then identifies the individual blocks and buildings as follows: (region) I, (block) I, (building) 1.
Regio I Forum Baths The Forum Baths (I, XII, 6) are located of the Via della Forica south of the Forum. The complex is also called the Thermae Gavii Maximi, named after Marcus Gavius Maximus, the praetorian prefect who sponsored their construction. The baths were intended for public use and covered c. 3,200 m2. Statues of Hygieia, Aesculapius, and Fortuna were some of the many statues recovered from the bath that most likely belonged to the Forum Bath's decorative program. Additionally, cipollino columns were used to decorate the bathhouse.
There were several phases of construction for the baths. The Forum Baths were initially built during the reigns of Emperor Antoninus Pius c. 160 with subsequent additions during the Severan Dynasty (193-225), reign of Maxentius and Constantine (306-337), and finally during the reign of Theodosius (379-395). The Baths were constructed in the city centre and were the largest of all Ostia baths with a large palaestra south of the main building. The Forum Baths are characteristic of the experimental mode of planning during Hadrianic-Antonine period. The structure may have reached 15–17 m tall, although not as high the remains are still impressive. Along the southern face of the building are the warm rooms of the bathhouse, placed to take advantage of the sun's passive heating. Here a modern visitor can see the octagonal sun-bathing room (heliocaminus), an elliptical sweating room (laconicum) two tepidaria and a caldarium with three pools. The palaestra was surrounded on three sides by porticos which shaded the entrance to shops. Many hairpins were recovered during excavations proving that the baths were used by women. It is assumed that this would be at a separate time of day from men, but no clear evidence exists.
Baths of Buticosus
This small bathhouse (I, XIV, 8) was constructed during the reign of Trajan circa 110 and remodeled in the middle of the second century This bath is typical of many of the balnea in Ostia, where the rooms are built into the established city grid leading to a chaotic interior layout often without a palaestra. In Room 4 is a black-and-white mosaic with marine animals and a man. Besides the man is an inscription EPICTETVS BVTICOSVS, giving the bath house its modern name. This man was a bathing attendant and holds a bucket and a stick. In the caldarium is another black-and-white mosaic with a marine scene of Triton and Nereid. The bath was supplied with water by a tank equipped with a noria in the adjacent Republican Sacred Area. These baths also preserved frescoes with garden images, creating an illusion of a real garden.
Baths of Mithras
The baths of Mithras (I, XVII, 2) were built around c. 125 C.E. and modified in the early Severan Period. The eponymous mithraeum was built in the late second or early third century C.E. and is accessible by a staircase in the northern part of the building. The remains preserve evidence of the utilitarian aspects of a Roman bathhouse such as a waterwheel for bringing water into the bathhouse and a boiler room for heating the caldaria and tepidaria. The frigidarium contains an apsidal pool and a mosaic showing Ulysses and the Sirens. Some of the earliest Christian imagery found in Ostia comes from the Baths of Mithras.
Small Baths This bath (I, XIX, 5)(c. ) was built in late antiquity around 450-500 C.E. reusing Hadrianic bricks. It was accessible only from the warehouse to the east of it. It is one of the last building projects before the abandonment of the city. The building was constructed above the rubble of the collapsed insulae and reused many of the rooms in the area for the bath such as the street entrance of a former shop being converted into the apodyterium.
Hadrianic Bath Under the Forum of the Heroic Statue (I, XII, 2) and the neighboring House the Cistern (I, XII, 4) are the remains of a bathhouse from the Hadrianic period. An octagonal room with niches is preserved in the House of the Cistern with evidence of a missing hypocaust system suggesting that this room was a heated part of the bath, although the later buildings have obscured much of the original use.
Regio II Baths of the Coachmen
This modest bathhouse (II, II, 3) is found close to the Porta Romana and Via Ostiensis. It has been speculated that it was the guild headquarters for the Cisiarii (Latin for Coachmen) however there is no direct evidence for this besides the subject of the mosaics. The structure was built during the Hadrianic era and refurbished during the 3rd Century CE. Notable is the large black-and-white mosaic in the frigidarium which depicts two sets of walls, possibly referring to Rome and Ostia, four figures of Atlas and a set of two-wheeled wagons (in Latin cisia) drawn by mules with silly names like Pudes (Prudish), Podagrosus (Gouty), Potiscus (Thirsty?), and Barosus (Effeminate). It is this mosaic that gives the name to the bathhouse. In recent excavations, a noria (wooden water wheel) was discovered in the rooms south of the frigidarium. Enslaved people would have walked in the wheel to draw up and distribute the water needed.
Baths of Neptune
East of the theater is another large square bath complex covering c. 4,400 square meters (67 meters x 67 meters) and is known as the Baths of Neptune (II, IV, 2). The structure, built late during the reign of the emperor Hadrian, was dedicated early (~139 C.E.) in the reign of Antonius Pius. At the end of the second century C.E it was restored by Gamala Iunior. The bath continued to be renovated until the 4th century C.E. The bath house is surrounded on four sides by streets and has entrances on each street. The overall design of the bathhouse was similar to those found in Pompeii, such as the Central Baths and therefore represents the next aspect of development.
Four large black-and-white figural mosaics were found here. The eponymous mosaic of Neptune riding a chariot drawn by hippocampi and surrounded by marine animals can be found after entering the bathhouse, this room is just south of the frigidarium. In an adjacent room is the mosaic of Amphitrite riding a hippocampus. The frigidarium has a mosaic of Scylla surrounded by sea divinities. The last figural mosaic is one of a group of athletes in a room to the south-east of the palaestra. There are two boxers with spiked gloves, two boxers-wrestlers, and a wrestler. None of the mosaics can be seen from any one position in the bathhouse, forcing a viewer to move through the space. The individuals in the mosaics have a three-dimensional quality to them, showing overlapping and twisting, although the scene as a whole remains flat.
The Baths of Neptune are one of the few in Ostia that contains an open-air palaestra, surrounded on three sides by marble columns. Under the palaestra was a reservoir that received water from the Julio-Claudian aqueduct and feed the bathhouses in the area. The bathhouse was constructed with a monumental facade along the Decumanus, built at the same time as the bathhouse and repaired in 350 C.E., possibly after an earthquake in 346 C.E. There is a central exedra paved with marble that had a life-sized statue of Sabina, wife of Hadrian, dressed as the goddess Ceres. The statue alludes to the role the imperial family had with this bathhouse and has been identified as a shrine for the Imperial Cult. The upper floors of the building had domestic dwellings with separate entrances.
Bath under the Via dei Vigili (Baths of the Provinces) A small bathhouse (II, V) has been partially uncovered in the Via dei Vigili, near the barracks of the firefighters. Some archaeologists also call it the Baths of the Provinces. The construction dates to circa 50 C.E. soon after Ostia received its aqueduct. The black-and-white mosaic is the most impressive part of the structure. At its center is a group of dolphins flanked by heads that represent different Roman provinces, encircling these are images of shields and spears implicating Rome's conquering of the provinces. These remains are the earliest archaeologists have recovered of a bath complex in Osita despite epigraphic evidence suggesting some of a late Republican date.
Regio III Baths of the Christian Basilica These baths (III, I, 2-3) were constructed during the reign of Trajan and restored sometime during the early third century C.E. It is unclear if they are related to their namesake, the neighboring Christian Basilica (III, I, 4).
Terme Marittime These baths (III, VIII, 2) were given the wrong name by early excavators at Ostia. Originally believed to be the Thermae Marittime in CIL XIV, 137 which has since been connected to the Porta Marina Baths. These baths were started around 130 C.E., destroying a part of the Republican city walls in their construction. They underwent two more sets of renovations around 210 C.E. and in the mid-4th century C.E. The bathhouse contains several mosaics with oceanic themes. One of them shows the head of Oceanus surrounding by four smaller tritons, done to emphasise the importance of the central figure. Beyond the typical bathhouse rooms, it also contains a set of terraces that led to the sea.
Baths of the Seven Sages
This bath complex (III, X, 2) creates a city block with houses to the north and the south (House of the Charioteers and House of Serapis) from the late Hadrianic/early Antonine period. It is possible this bathhouse was only intended for the residents of the two adjoining insulae. The bath is named after a satirical painting of seven Greek philosophers found in a room that was formerly a tavern and later used as the apodyterium, the philosophers are identified by their names and city of origin written in Greek and a humorous and ironic Latin inscription referring to latrine activities. The central hall contains a mosaic with 5 concentric rings depicting hunting scenes. The room adjacent to the natatio is a fresco of Venus Anadyomene.
Baths of Trinacria These baths (III, XVI, 7) which were built at the same time as the Serapeum, were constructed during the reign of Hadrian. They were named after a mosaic found in the corridor south of the frigidarium which shows a female head with a triskeles, or three legs, behind her. The suspensurae, or raised floor, was restored during the reign of Commodus. In one of the heated rooms is a mosaic that potentially advertised where one could find prostitutes in the bathhouse. Although others suggest that it was a nickname for an unofficial youth organization.
Baths in Horrea III, XVII, 1 This warehouse was built during the Hadrianic Period and most likely was privately owned. During the fourth century C.E. a set of rooms in the south end of the building was converted into a bath suite with mosaics. This balnea, along with several other refurbished bathhouses, served the Late Antique population of Osita.