"Almost a century after Caracalla gave Romans his gargantuan Baths, Emperor Diocletian, who never even visited Rome, strove to outshine his imperial predecessor by commissioning the largest and most gorgeous bathing establishment the world had ever seen.
It could accommodate 3000 bathers simultaneously, about twice as many as the Baths of Caracalla, covered 13 hectares (32 acres) and had the full panoply of changing rooms, gymnasiums, libraries, meeting rooms, theaters, concert halls, sculpture gardens, vast basins for hot, lukewarm and cold plunges, as well as mosaic floors and marble facades. Today's luxurious spas and health resorts are but pale copies of the Baths of Diocletian.
Fragments of the Baths' core were incorporated into the Renaissance Church of Santa Maria degli Angeli by Michelangelo and now form part of the Museo Nazionale Romano.
The Baths were built of brick that was faced on the inside with marble and on the outside with white stucco imitating blocks of white marble, like the Baths of Caracalla. The enormous central hall, 280 by 160 yards, is an engineering wonder that was the model for the Basilica of Maxentius in the Roman Forum." - Roma Online