GCANS Metropolitan Area Outer Underground Discharge Channel is an underground water flood control project is the largest underground flood water diversion facility in the world. The facility was built to control the overflow of Tokyo's rivers during rain and monsoon seasons. GCANS has five giant containment silos connected with six kilometers of tunnels. Its most famous feature is the large holding tank cistern with giant pillars that resemble an underground cathedral or temple . The main job of GCANS is to store then divert overflow of water into the Edo River to avoid flood damage.