Among the most significant and remarkable remains are several domed baths buildings such as the great so-called Temple of Mercury, the Temple of Venus, and the Temple of Diana. They were traditionally credited to some of the more famous residents of the town's villas, although they were not temples but parts of thermal baths.

The public and private baths of Baiae were filled with warm mineral water directed to their pools from underground hot springs, as many still are today. Roman engineers were also able to construct a complex system of chambers that channelled underground heat into facilities that acted as saunas. In addition to their recreational function, the baths were used in Roman medicine to treat various illnesses and physicians would attend their patients at the springs.

circa

Foundation
Baiae was constructed on the Cumaean Peninsula within the Phlegraean Fields, an active volcanic region. It occupied the side of an ancient crater that sloped gradually down to the shoreline. The town may have originally developed as the port for Cumae, known as the Portus Baianus. Its location provided it with natural thermal springs, which were quickly exploited for the establishment of luxurious bathing complexes.

Early Roman Period (80 BCE- 1st Century CE)

By around 80 BCE, Baiae had transformed into a highly fashionable retreat for Rome’s aristocracy. Prominent figures such as Gaius Marius, Lucius Licinius Lucullus, and Pompey the Great were known to frequent the resort. Julius Caesar also owned a villa there, and under Augustus, much of the town became imperial property, further enhancing its prestige and exclusivity.

From 36 BCE, Baiae included Portus Julius, the base of the western fleet of the Roman Navy. This naval facility was initially of great strategic importance, but it was eventually abandoned due to the silting of Lake Lucrinus. A short channel from the lake had once connected it to Lake Avernus, but as sediment accumulated, the harbor at Portus Julius became unusable. The fleet was subsequently moved to the two harbors at Cape Miseno, located approximately four miles (6.4 km) to the south.

Even during the late Republic, Baiae had already gained a notorious reputation as a center of indulgence and excess. In 56 BCE, the prominent socialite Clodia was vilified during the trial of Marcus Caelius Rufus, where she was accused of living as a harlot in Rome and spending her time in Baiae engaging in scandalous behavior, including extravagant beach parties and excessive drinking. The Augustan-era poet Sextus Propertius described the town as a "den of licentiousness and vice." In the 1st century CE, Seneca the Younger devoted one of his moral epistles to denouncing the town’s decadence, labeling it a "vortex of luxury" and a "harbor of vice," where young women acted frivolously, elderly women sought to reclaim their youth, and certain men indulged in behavior deemed unbecoming.

Imperial Period (1st- 2nd Century CE)
Several emperors were closely associated with Baiae. Nero, known for his opulent tastes, commissioned the construction of a grand villa there in the mid-1st century CE. Hadrian, another imperial patron of the resort, spent his final days in Baiae, dying at his villa in 138 CE. The emperor Septimius Severus was also particularly fond of the town, which continued to attract Rome’s elite well into the Imperial period. The resorts actively capitalized on their connections to the imperial family; according to Suetonius, as late as 120 CE, visitors could view the cloak, brooch, and gold bulla once given to the young Tiberius by Pompey’s daughter, Pompeia Magna.

Baiae was also the stage for one of the most infamous spectacles of the early empire. According to Suetonius, in 39 CE, the eccentric emperor Caligula staged an elaborate stunt to challenge the astrologer Thrasyllus, who had once predicted that he had "no more chance of becoming emperor than of riding a horse across the Gulf of Baiae." In response, Caligula ordered the construction of a massive pontoon bridge stretching three miles across the gulf, connecting Baiae to the nearby port of Puteoli. Using impounded ships weighted with sand, he formed a continuous roadway over the water. Clad in a gold cloak, he then rode across the bridge on horseback in a display of both extravagance and defiance. The historian Cassius Dio adds that Caligula even ordered resting places and lodging stations with potable water to be set up along the bridge. As late as the 18th century, scattered remnants of this remarkable structure were still pointed out to visitors as the "Bridge of Caligula." Some scholars, such as Malloch, have argued that Suetonius's portrayal of the event was colored by his bias against Caligula, suggesting instead that the bridge was a calculated attempt to establish military prestige and legitimacy.

Despite its prominence, Baiae never attained municipal status and remained under the administration of nearby Cumae.

Late Roman Period and Decline (4th- 6th Century CE)
By the late 4th century CE, the town still attracted the Roman elite. The immensely wealthy consul Quintus Aurelius Symmachus, who owned multiple villas across the Bay of Naples, maintained a grand estate at Baiae known as the praetorium. He also wrote fondly of his villa at Bauli (modern Bacoli), located nearby, which he praised for its tranquility and scenic views.

In the early 6th century CE, the Ostrogothic king Athalaric expressed admiration for the natural beauty of Baiae. He was particularly taken with its picturesque bay, the abundance of oysters, and the renowned baths fed by geothermal springs, believed to have therapeutic properties.

The gradual subsidence of the land, a phenomenon known as bradyseism, led to the partial submersion of Baiae over time. This process seems to have occurred in two phases: an initial lowering of the coastline between the 3rd and 5th centuries CE, during the late Imperial period, followed by a more significant submersion approximately a century later. By the 8th century, much of the lower part of Baiae had been engulfed by the sea.

Medieval Period (8th- 16th Century CE)
Throughout late antiquity and the early medieval period, Baiae suffered repeated invasions and attacks. It was sacked during the barbarian incursions and later targeted by Muslim raiders in the 8th century. By 1500, the town had been entirely abandoned due to recurrent outbreaks of malaria, leaving its once-grand structures in ruins. However, during the 16th century, the Spanish viceroy Pedro de Toledo recognized the site's strategic significance and constructed the Castello di Baia, a fortress that still stands today as a testament to the area's layered history.

circa 80 BCE

Villa of the Ambulatio
The Villa of the Ambulatio (Villa dell'Ambulatio) overlooks the sea and features six cascading terraces connected by an intricate network of staircases, the final of which leads to the "Sector of Mercury". It is named after its ambulatio, a long covered walkway (inspect) with two longitudinal naves on the second terrace, offering sweeping views of the gulf. Traces of fine stucco remain on the brick pillars of this structure. The upper terrace housed the villa’s residential quarters, once richly decorated and designed for leisure. The third terrace has since been converted into a garden, while the fourth was designated for service areas. The fifth terrace contained rooms likely meant for relaxation, opening toward the sea and leading to the lowest terrace, which, like today, was originally a garden, possibly enclosed by a colonnade.

circa 50 BCE

Temple of Mercury
The so-called "Temple of Mercury", built in the 1st century BCE during the late Roman Republic, features a 21.5 meters (71 feet) diameter dome, the largest of its time and the oldest known surviving concrete dome. Constructed with large tuff blocks, it includes an oculus and four square skylights, allowing natural light to filter in. Originally, it enclosed the frigidarium, or cold pool, of the public baths. Eighteenth-century descriptions suggest it once had six niches, four of which were semicircular.

circa 50 BCE- 50 CE

Small Thermal Baths

circa 50-60 CE

Sector of Aphrodite Sosandra
The so-called sector of Sosandra, also called "Temple of Sosandra", named after a statue of Aphrodite Sosandra (inspect) discovered in 1953 CE and now housed in the National Museum of Naples, is a multi-terraced sector framed by two parallel staircases. It consists of a villa, a spa, and a hospitalia—an accommodation for visitors to the nearby baths. According to Cassius Dio, this complex corresponds to the ebeterion built by Nero for the relaxation of sailors from the Misenum fleet. The highest terrace contains service areas and a small balneum with elaborately decorated stucco ceilings. Below it, a large terrace enclosed on three sides by a portico overlooks a garden, where four parallel walls may have defined three outdoor triclinia. Above the peristyle, several residential chambers were richly adorned, particularly with valuable mosaic floors depicting theatrical masks framed within geometric patterns. Further down, a semicircular structure with five vaulted rooms was once concealed behind a decorative façade of niches and columns, creating an imposing visual effect. At the heart of the complex, a room likely served as a nymphaeum, supplying water to a large external circular basin. The lower terrace peristyle preserves wall paintings from two periods: mid-1st-century CE frescoes with Egyptian motifs related to the cult of Isis, later overlaid in the 2nd century CE by architectural compositions featuring male and female figures.

circa 117-138 CE

Hadrian's Baths

circa 125 CE

Temple of Venus
This domed structure was the central hall of a vast bath complex, likely commissioned by Hadrian, as indicated by its architectural similarities to his Villa at Tivoli. Externally octagonal and internally circular with a 26-meter diameter, it featured eight large arched windows and a balcony overlooking the pool. The name "Temple of Venus" arose from a misidentified statue found there. Its advanced umbrella-shaped dome, composed of sixteen alternating spherical and veloidic segments, was an unprecedented innovation before Hadrian, with parallels in the Piazza d’Oro and Serapeum at Tivoli. Constructed from light tuff and Vesuvian pumice, like Hadrian’s Pantheon, its supporting drum was decorated with stucco imitating marble blocks with blue glass-filled joints, creating a shimmering effect. The forecourt had an intricate geometric design, echoing Hadrianic architecture, while the interior was richly adorned with mosaics, and the pool and lower walls were clad in marble.

circa 222-235 CE

Temple of Diana
The partially collapsed ogival dome, originally part of a thermal bath complex, once collected geothermal vapors and was adorned with marble friezes depicting hunting scenes. Identified as a structure built by Alexander Severus (reigned 222–235 CE) in honor of his mother, Julia Mamaea, it may have served as a dynastic heroon. In 1677, Cornelis de Bruijn described the ruins, noting the remains of the so-called Temple of Diana, a semicircular structure still standing, and nearby remnants of the Temple of Apollo and a chamber of nymphs adorned with bas-reliefs and figures, which he found both strange and beautiful.

circa 250-800 CE

Underwater Archaeological Park
The Underwater Archaeological Park of Baiae is one of the most extraordinary submerged cultural sites in the world, preserving the ruins of the once-lavish Roman resort town, much of which now lies beneath the Bay of Naples due to bradyseism—a gradual subsidence caused by volcanic activity. Stretching across an area of approximately 177 hectares, the park encompasses the remnants of opulent villas, marble-paved roads, bath complexes, mosaics, and statues that once adorned the palatial retreats of Rome’s elite. The structures, submerged between the 3rd and 8th centuries CE, remain remarkably well-preserved beneath the shallow waters, offering an unparalleled glimpse into the luxurious lifestyle of imperial Rome. Visitors can explore the site through guided scuba diving or glass-bottom boat tours, revealing intricate floor mosaics, columns, and even well-defined rooms, some of which still bear their original frescoes.

Among the most famous features of the park are the so-called Nymphaeum of Claudius, an ornate banqueting hall with statues representing members of the Julio-Claudian dynasty, and the Villa a Protiro, a residence with a stunning black-and-white mosaic atrium. The remains of Portus Julius, the once-grand naval base of the Roman fleet, also lie within the submerged area, testifying to Baiae’s military significance alongside its reputation as a hedonistic retreat. The slow-motion sinking of the city has ironically preserved much of its architecture, sparing it from later human destruction and looting. Today, the Underwater Archaeological Park of Baiae serves as both a unique dive site and a crucial historical record of Rome’s coastal engineering, artistic grandeur, and volcanic vulnerability.