What Roman public baths in Rome were the largest, built under Diocletian?

The largest Roman imperial public baths in Rome, built under Diocletian, were the Baths of Diocletian.

Construction began in 298 AD at the order of the emperor Maximian, who dedicated the complex to his co-emperor Diocletian. Work continued into the early fourth century and was completed around 306 AD. The baths occupied roughly 13 hectares and served residents of several districts around the Viminal, Quirinal, and Esquiline hills.

Like other Roman thermae, the complex included hot, warm, and cold bathing areas, traditionally called the caldarium, tepidarium, and frigidarium. It also had exercise courts, libraries, gardens, monumental halls, and elaborate decoration. Ancient sources claimed that as many as 3,000 people could use it at once, though that capacity estimate is disputed.

The baths remained in operation until about 537, when the Ostrogoths cut Rome’s aqueducts during a siege. Later buildings incorporated the ruins, most famously the Basilica of Santa Maria degli Angeli e dei Martiri, designed by Michelangelo in the sixteenth century. Parts now belong to the National Roman Museum.