In a word, the Baths of Caracalla are enormous. Towering 125 feet above you, they're higher than most apartment complexes in modern-day Rome; they accommodated up to 1,600 people at a time. Today, however, there's hardly anyone there.
The baths (or terme, in Italian) date back to the early 200s A.D., when they were planned by Emperor Septimus Severus and completed by his son Caracalla. They boasted a 183-by-79-foot frigidarium, tepidarium, 115-foot-wide caldarium, natatio (swimming pool) complete with bronze mirrors to reflect in the sunlight, and two palaestras, or gyms. And that's not to mention the complex's dozens of shops and two public libraries — one with texts in Latin, one in Greek. 
Today, the baths' sheer size is enough to take your breath away. Experiencing that enormity and, with it, getting a sense of the monumental scale of ancient Rome's structures, is alone an excellent reason to visit. Unless you have a fascination with ancient baths, it's also pretty much the only reason: After centuries of looting and plundering, the baths are just a shell of what they would have been.
As with so many of Rome's other ancient sites, imagining the baths as they would have been requiressome imagination. (The History Channel has a short online video on the baths including a reconstruction, which helps; here is a scale drawing of the complex). If you go, remember that the entire bath complex would have been lavished with glass-paste mosaics (like that above), frescoes, and 
hundreds of sculptures.
Two of the sculptures, unearthed in the 16th century during excavations by Pope Paul III Farnese, give an idea of how monumental and lavish these decorations would have been: the Farnese Hercules and the Farnese Bull, an enormous sculpture group carved from one marble block that was probably originally a fountain (right), both now in Naples.
Visiting the baths costs € 6.00 for non-E.U. citizens; the ticket is also good for 7 days for Villa dei Quintili and the Mausoleo di Cecilia Metella. The complex is open daily from 9am to the evening (check the official Baths of Caracalla website for exact times, which vary by season), and to 2pm on Mondays. They're located just southeast of Circus Maximus, which is the closest metro stop. For a map, click here. To plot your route with public transport (official address is Viale delle Terme di Caracalla, 52), click here.
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