The city of Bath grew up around Britain's only natural hot spring, where roughly 1.17 million litres of geothermally heated water rise to the surface every day at about 46°C. The Romans built a temple and bathing complex, Aquae Sulis, around the spring — channelling the water into the Great Bath and heating adjoining rooms with a hypocaust: a raised floor on stacks of tile pillars (pilae) through which hot air from a furnace circulated. Centuries later the ruins silted up and vanished, and Georgian Bath was built directly on top of them, roughly 6 metres above the Roman street level — until the buried baths were rediscovered in the 1870s.
The spring water fell as rain on the Mendip Hills up to 10,000 years ago, sinking deep underground before being forced back up along a geological fault — meaning the water bubbling into the Great Bath today began its journey before the pyramids were built.
A 3D cutaway of the Roman Baths at Aquae Sulis, showing the hot spring rising through a geological fault to fill the Great Bath and feed a hypocaust hot-air heating system — buried six metres beneath the Georgian city that was later built directly on top of it.
Geothermal water rises along a fissure into the Great Bath while a furnace-driven hypocaust pushes hot air under a raised floor and up through wall flues to heat the caldarium — the two heat sources that made the Roman spa function.
Drag the "Excavate through time" slider to lift the Georgian street, Pump Room and terraces away and reveal the Roman ruins below, then adjust the spring flow rate and hypocaust fire to see the heating systems respond.
The Roman baths were lost for centuries under silt and Georgian foundations, only rediscovered by Victorian engineers investigating a leak in the 1870s — the same layered history this simulation lets you dig through.