4 Best Sights in Aventino and Testaccio, Rome

A decommissioned early-20th-century power plant is now this intriguing exhibition space for the overflow of ancient art from the Musei Capitolini collection. Getting here is half the fun. A 15-minute walk from the heart of Testaccio iwill lead you past walls covered in street art to the urban district of Ostiense. Head southwest and saunter under the train tracks passing buildings adorned with four-story-high murals until you reach the often-uncrowded Centrale Montemartini, where Roman sculptures and mosaics are set amid industrial machinery and pipes.

Unusually, the collection is organized by the area in which the ancient pieces were found. Highlights include the former boiler room filled with ancient marble statues that once decorated Rome's private villas, such as the beautiful Esquiline Venus, as well as a large mosaic of a hunting scene.

Although the line to peek through the keyhole of a nondescript green door in the Gran Priorato, the walled compound of the Knights of Malta, sometimes snakes around Piazza dei Cavalieri di Malta, the enchanting view is worth the wait. Far across the city, you'll see the dome of St. Peter's Basilica flawlessly framed by the keyhole and tidily trimmed hedges that lie just beyond the locked door. The priory and the square are the work of Giovanni Battista Piranesi, an 18th-century engraver who is more famous for etching Roman views than for orchestrating them, but he fancied himself a bit of an architect and did not disappoint.

Founded in the Holy Land during the Crusades, the Knights of Malta is the world's oldest and most exclusive order of chivalry. The knights amassed huge tracts of land in the Middle East and were based on the Mediterranean island of Malta from 1530 until 1798, when Napoléon expelled them. In 1834, they established themselves in Rome, where ministering to the sick became thelr raison d'être. Private, guided tours of the Gran Priorato are usually offered on Friday morning, but you must prebook by email.

One of Rome's oldest churches—built in the 6th century and restored in the late 19th century—is on the Piazza della Bocca della Verità, originally the location of the Forum Boarium, ancient Rome's cattle market and later the site of public executions. Although the church has a haunting interior and contains the flower-crowned skull of St. Valentine, who is celebrated every February 14th, it plays second fiddle to the renowned artifact installed out in its portico.

The Bocca della Verità (Mouth of Truth) is in reality nothing more than an ancient drain cover, unearthed during the Middle Ages. Legend has it, however, that the teeth will clamp down on a liar's hand if they dare to tell a fib while holding their fingers up to the fearsome mouth. Hordes of tourists line up to take the test every day (kids especially get a kick out of it).

The Terme di Caracalla are some of Rome's most massive—yet least visited—ruins. Begun in AD 206 by the emperor Septimius Severus and completed by his son, Caracalla, the 28-acre complex could accommodate 1,600 bathers at a time. Along with an Olympic-size swimming pool and baths, the complex also had two gyms, a library, and gardens. The impressive baths depended on slave labor, particularly the unseen stokers who toiled in subterranean rooms to keep the fires roaring in order to heat the water.

Rather than a simple dip in a tub, Romans turned "bathing" into one of the most lavish leisure activities imaginable. A bath began in the sudatoria, a series of small rooms resembling saunas, which then led to the caldarium, a circular room that was humid rather than simply hot. Here a strigil, or scraper, was used to get the dirt off the skin. Next stop: the warm(-ish) tepidarium, which helped start the cool-down process. Finally, it ended with a splash around the frigidarium, a chilly swimming pool.

Although some black-and-white mosaic fragments remain, most of the opulent mosaics, frescoes, and sculptures have found their way into Rome's museums. Nevertheless, the towering walls and sheer size of the ruins give one of the best glimpses into ancient Rome's ambitions. If you're here in summer, don't miss the chance to catch an open-air opera or ballet in the baths, put on by the Teatro dell'Opera di Roma.