The Sewers were a network of tunnels beneath the United States on Earth, designed to transport waste. Xaq made use of them several times. On his first adventure, he escaped from everyone trying to kill him through the sewers. There, he met Laura and rescued Colette for the first time. (Series 1: "The Sewers")
Much later, Zoovac informed Xaq of a passage through the sewers that might lead to the lost city of Atlantis. Unfortunately, members of The Company had discovered it as well, and raced Xaq to discover the ancient city. (Series 1: "Atlantis Quest")
Years later, Pierre and Ari, having infiltrated the House of Rorric in Germany, escaped through the Sewers after rescuing Colette and Zoovac. (Pierre Series: "The Evil League of Evil-Ness")