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.
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.