Baths of Caracalla: Hypocaust Tunnels, a Blood Pit Mithraeum, Penn Station

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Built between about 212 and 216 AD under Septimius Severus and his son Caracalla, the Baths of Caracalla took some 2,000 tons of stone, brick and marble a day for six years. The complex covered 25 hectares, about 62 acres, and handled 6,000 to 8,000 bathers daily, with gardens, two palaestras for boxing and wrestling, and separate Greek and Latin libraries. The frigidarium's groin vaults rose almost 33 meters on Egyptian granite columns weighing 100 tons each, and the caldarium's dome, nearly 36 meters across, rested on eight pillars with glass windows that trapped solar heat. Bronze mirrors lit the 50-meter natatio, and a mirrored plan on a northeast to southwest axis funneled crowds evenly toward the central baths.

The baths sat on a six-meter platform above hundreds of meters of tunnels. The Aqua Antoniana delivered water, furnaces sent heat under floors raised on brick pillars and up hollow wall tiles, and slaves burned about 10 tons of wood a day from stores of over 2,000 tons, beside an underground water mill and the sewers. In 1912 excavators found the largest known Mithraeum, with a pit many scholars read as a fossa sanguinis where initiates were drenched in bull's blood. The Ostrogoths under Vitiges cut the aqueducts in 537, an earthquake brought down vaults in 847, and Renaissance popes quarried the ruins, from Pius II's loggia at St. Peter's to Pope Clement VII's 1524 license for Cardinal Lorenzo Pucci.

• Pope Paul III's Farnese excavations recovered the Farnese Bull and the Farnese Hercules, now in Naples.
• Two granite tubs from the frigidarium became the fountains of Piazza Farnese.
• Charles McKim modeled the main waiting room of New York's 1910 Pennsylvania Station on the frigidarium.
• The ruins hosted gymnastics at the 1960 Olympics and the first Three Tenors concert in 1990.
• The site appears in films from La Dolce Vita to John Wick: Chapter 2.