They were also used to store the fuel for the furnaces – which are estimated to have burned around 10 tons of wood per day. Storage capacity had been estimated at more than 2,000 tons of wood. Overall, these tunnels ran for hundreds of meters underneath the main structure. They were lit by skylight-type windows that allowed the air to circulate, thus keeping the stored wood from rotting. An underground water mill was excavated beginning in 1912. First thought to be medieval in origin it is now considered to date to Antiquity. It may have been a part of the original workings of the baths. Fire damage indicates it was burned in the 3rd century, but rebuilt with some alterations. The columns of the "frigidarium" were made of granite and they weighed close to . The baths were originally ornamented with high quality sculptures, estimated to have numbered more than 120. Despite their location in one of the city's working class areas, of all the antique baths in Rome the Baths of Caracalla were found to have contained the most lavish assortment of statues. Although many were destroyed in the Middle Ages to make lime, beginning in the 16th century under Pope Paul III Farnese, sculptures were excavated from the area to serve as decorations in newly built palazzi. Among the well-known pieces recovered from the Baths of Caracalla are the "Farnese Bull" (probably from the eastern "palaestra") and "Farnese Hercules" (from the "frigidarium"), now in the Museo Archeologico Nazionale, Naples; others are in the Museo di Capodimonte there. One of the many statues is the colossal statue of Asclepius. Two granite tubs from the "frigidarium" were taken to Piazza Farnese where they still serve as fountains, made by Girolamo Rainaldi. A granite column from the "natatio" was taken to Florence by Cosimo I de' Medici in Piazza Santa Trinita. It is now known as the Column of Justice. In the Lateran Collection of the Vatican Museums are large mosaics of rectangular panels representing athletes, which were discovered in 1824 in two of the library exedrae to the east and west of the baths complex. They have been restored and are on display in the Gregoriano Profano Museum. The central part of the bath complex (the "caldarium") was the summer home of the Rome Opera company from 1937 to 1993.