In the middle of a patch of grass amid the ruins of the Caracalla baths in Rome, there is a staircase that takes visitors deep into the ground to a world resembling the lair of a James Bond villain.

“This is our glimpse at maniacal Roman perfection, at incredible hydraulic technology,” said archaeologist Marina Piranomonte, as she descended and waved at a network of high and wide tunnels, each measuring six metres (20ft) high and wide, snaking off into the darkness.

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The baths, on a sprawling site slightly off the beaten track in a city crowded by monumental attractions, hold their own against the nearby Circus Maximus, its shattered walls standing 37 metres high, recalling its second century heyday when it pulled in 5,000 bathers a day.

But for Piranomonte, it is the three kilometre, triple-tiered grid of tunnels that lies under the site – the first tract of which will open for visits this month – which really shows off how seriously the Romans took their sauna time.

An army of hundreds of slaves kept firmly out of sight of bathers scurried along the tunnels feeding 50 ovens with tonnes of wood a day to heat water surging through a network of underground channels that arrived via aqueduct from a source 100km away. Below that, massive sewers, which are now being explored by speleologists, flowed towards the Tiber.

“It’s the dimension and the organisation that amazes – there is no spa as big as this anywhere in the world today,” said Piranomonte.

Upstairs, Romans would kick off a visit with a session in one of two gyms, then enjoy a sauna and a spell in a hot tub in the 36 metre (120ft) wide, domed caldarium – slightly smaller than Rome’s Pantheon. The tepidarium then beckoned, before a cool down in the frigidarium, a space so elegant its design and dimensions were copied at Union station in Chicago.

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“The side room at the station where the shoot-out on the stairs is set in The Untouchables actually contained a large cold bath here,” said Piranomonte.

To complete the experience, a pool 50 metres long and a garden complete with lending library flanked the baths. “The emperor Caracalla was cruel, but he built beautiful things,” said Piranomonte, who is charged with the site’s upkeep.

A thousand years after it was built, the ghostly ruins of the massive buildings were overgrown and abandoned. “Because it was on the outskirts of Rome, no one built on top of it and the tunnels were simply forgotten, probably sealed by undergrowth,” she added.

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Following their rediscovery at the end of the 19th century, Mussolini strengthened the tunnels when he decided to stage operas amid the ruins overhead, but Piranomonte was less than impressed with his handiwork.

“Look at the rain water trickling through; that’s Mussolini’s bricks leaking while ours are fine,” she said, pointing to the perfect Roman brick arches disappearing into the gloom.

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The reopening of a short stretch of the tunnels on 21 December caps a clean-up of the baths. The opera, which used the remains of the caldarium for a stage and kept a stage-set workshop in one of the saunas, has been shunted back into the gardens.

A €450,000 (£360,000) restoration programme also resulted in the reopening this month of an underground temple at the baths, linked to the tunnel network and dedicated to Mithras, the deity whose popularity soared just before Christianity took hold in the Roman empire. Entering the temple, which boasts black-and-white floor mosaic and is the biggest of its kind in the Roman empire, Piranomonte points to a frieze of Mithras holding a globe but missing his head. “Probably taken off by the Christians,” she said.

A chamber flanked by space for spreading out on during banquets centres on a large pit where a drugged bull was placed on a metal grill and butchered. Below the grill is a small niche where an initiate to the cult would crawl to be drenched with litres of bull’s blood. “It was a cruel cult, for men only, so you understand why Christianity got the upper hand,” said Piranomonte.

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Emerging from the temple, the archaeologist turns left and pauses before what she describes as her favourite part of the baths – an authentic Roman roundabout. A large arch leads to the entrance of the tunnel network, where carts carrying tonnes of logs would queue to enter to feed the ovens. Now fully excavated and restored, the tunnel starts with a roundabout that circles a guard’s kiosk to stop traffic jam.

“A Roman spa with a roundabout,” said Piranamonte, “That I find really fascinating.”

guardian.co.uk © Guardian News and Media 2012

[Baths of Caracalla in Rome via Shutterstock]