Hot springs have been used for bathing at least since paleolithic times[11] The oldest known spa is a stone pool on China's Lisan mountain built in theQin Dynasty in the 3rd century BC, at the same site where the Huaqing Chi palace was later built. In the first century AD, Romans conquered Aquae Sulis, now Bath, Somerset, England, and used the hot springs there to feed public baths and underfloor heating. The admission fees for these baths probably represent the first commercial use of geothermal power. The world's oldest geothermal district heating system in Chaudes-Aigues, France, has been operating since the 14th century.[12] The earliest industrial exploitation began in 1827 with the use of geyser steam to extract boric acid fromvolcanic mud in Larderello, Italy.