The Baths of Trajan, begun in AD 104 AD, were a massive bathing and leisure complex. Much like the Baths of Titus, those of Trajan covered some of ruined golden palace of Nero (Domus Aurea).
The massive underground cistern, capable of storing no less than 8 million litres, is still there, known as the sette sale. Although they remain closed to the public.
Model of Rome in the days of Constantine the Great
Museo della Civilta, Rome