During the 3rd century A.D., Rome’s Baths of Caracalla welcomed some 5,000 visitors daily. The enormous complex—the second-largest in the history of the Roman Empire—included three bathing sections of varying temperatures, a natatio or swimming pool, two gyms for wrestling and boxing, restaurants, libraries, and even waxing salons.

Beneath the baths’ sumptuous aboveground attractions, slaves toiled in a roughly 2-mile-long, 6-mile-wide network of underground tunnels, transporting carts of wood to the 50 brick ovens responsible for keeping the building’s caldarium (a domed room containing seven 39-foot plunging pools) at a constant temperature of 104 degrees Fahrenheit.

A stretch of this labyrinthine opened to the public in 2012, but as Tom Kington reports for the Times, a section of the tunnels revealed earlier this week is the first to feature one of the surviving brick ovens.

“This is the technological heart of the baths,” Marina Piranomonte, director of the site, tells Kington. “Everyone should see it—not just academics with torches” (or flashlights, as they’re called on this side of the Atlantic).

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The Baths of Caracalla survived for more than 300 years, only closing in 537 A.D. after invading Witigis forces destroyed the city’s water supply. Eventually, the imposing complex was stripped of its marble and fell into ruin. Thanks to the $392,000 USD restoration campaign, however, tourists can now explore the space, roaming the baths’ still-standing walls and the extensive network of tunnels hidden below.