Built as a show of strength for the namesake emperor, this forum’s claim to fame is the Temple of Mars Ultor, with parts of the podium and supporting columns still standing. Though they’re technically part of the sprawling Roman Forum complex (Foro Romano), these ruins can be seen simply by walking along Via Tor de’ Conti—or during evening light projection shows from several vantage points in the neighborhood.

Once the second-largest bathhouse in ancient Rome, the 25-acre (10-hectare) site holds the ruins of a public spa once serving thousands of bathers daily. A truly vast complex with gyms, gardens, and libraries, the bathhouse can be visited independently; however, guided skip-the-line tickets are recommended, as the bathhouse is best seen with guides giving insight into everyday Roman life during Caracalla’s heyday. During summer, the baths host music and dance performances alfresco.

Set along the early stages of the Appian Way, these underground burial chambers beneath a 4th-century basilica were the first to be called “catacombs,” and once safeguarded the remains of Saints Peter and Paul. Home to period artworks, three mausoleums, and haunting engraved invocations, the catacombs are often seen on tours paired with visits to the Catacombs of Saint Callixtus, a nearby network of underground galleries where 16 popes have been buried.

Located on the Via dei Fori opposite the Colosseum, Trajan’s Market is often referred to as the world’s oldest shopping mall, though debate exists about whether the site’s arcade structures were designed for shops or administrative offices. An adjacent museum houses archeological artifacts and serves as a window onto the market’s former great hall—explore this area before viewing the ruins from the Imperial Roman Forum complex.

Rome’s grand Piazza Navona sits atop the ruins of the Stadium of Domitian (Stadio di Domiziano), an ancient 30,000-seat stadium that once hosted public athletic games. After checking out Navona’s grand palazzi and famous Bernini fountain, head to the ruins’ subterranean entrance on Via di Tor Sanguigna—below the piazza you’ll find the UNESCO-listed remains of one of Rome’s most popular underground ruins.

Five chapels underneath the Church of Santa Maria della Concezione are adorned with elaborate sculptures made from the bones of Capuchin monks. Perhaps Rome’s most haunting religious sites, the ossuaries are decorated with skulls, leg bones, pelvis bones, and mummified monks in friars' clothes. Skip strait-laced St. Peter’s and join a guided tour of the Capuchin museum and crypt, sit in the chapels for a concert of sacred music, or tempt the spirits by touring the chapels at night.

Skirting the outer edges of southeastern Rome, the Aqueduct Park is home to the ruins of Aqua Felix and Aqua Claudia aqueducts—the latter is much older and considered one of the great aqueducts of Rome. Though technically part of the Appian Way, the park is virtually tourist-free. To explore the park, cycle over from the Appian Way on e-bikes, join off-the-beaten-path walking tours, or see it along with Rome’s other hidden gems.