The first section of tunnels beneath Rome’s Baths of Caracalla  will soon open to tourists. Slaves used the tunnels to carry wood to the 50 ovens heating water for thousands of bathers a day. “It’s the dimension and the organization that amazes—there is no spa as big as this anywhere in the world today,” said archaeologist Marina Piranomonte. Below the tunnels, sewers carried waste water to the Tiber River, and men gathered and butchered bulls in an underground temple dedicated to Mithras.