Rome — There’s a reason archaeologist Ersilia D’Ambrosio can scarcely contain her excitement as she leads the way through dimly lit passageways deep below the Capitoline Hill that was once at the heart of ancient Rome: In a city where almost every historic treasure has been laid bare, this vast subterranean labyrinth is an undiscovered world.
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“No one has seen these caves and tunnels for more than a century,” D’Ambrosio tells CNN, plunging further into the gloom. These chambers, which cover around 42,000 square feet, or 3,900 square meters — roughly three-quarters the area of an American football field — lie in an area beneath the Ancient Roman Forum and the 2,000-year-old Marcello Theater. At its deepest point, one of the caves extends about 985 feet below the surface.
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Known as the Grottino del Campidoglio, or Capitoline Grotto, these tunnels have been part of the fabric of Rome even since before the days of Julius Caesar, despite being forgotten in recent generations. Comprehensively developed in the Middle Ages, they were in continuous use until the 1920s, at various times housing entire communities, shops, taverns, restaurants and, in World War II, people sheltering from falling Allied bombs.

Above ground, on the steamy morning in July when CNN was granted exclusive access to the cavern network, tourists sweated in temperatures of 95 degrees Fahrenheit (35 Celsius) as they explored the Capitoline Square, designed by Michelangelo in the 16th century, and the Capitoline Museums complex. Seventy-five feet below, in the grotto, it’s decidedly cooler at around 55F, with the damp air causing condensation to glitter on some of the tunnel surfaces.
Some of the passages are neatly constructed and lined with bricks, a sign of their development and use in the 19th century. Others are more roughly hewn from tuff, a soft volcanic rock from which the famous Seven Hills of Rome are formed. Walking through the tunnels is a trip back in time, with Rome’s complex layers of history laid bare.