The natural warm water outflow from the Gotthard Base tunnel has to be cooled down before it can be disposed in the local river Ticino. Geothermal swimming pools in an old factory hall are proposed to make use of the water and to cool it down. The front section of the old Monteforno steel factory serves as a multi purpose hall and the location for thermal swimming pools. The baths are placed on mid-level inside of the factory and consist of a thermal pool, a sport pool, a diving pool and a leisure pool. Bellow the pools, inside the column, the space is used to house a bar, a restaurant and a small shop. Inside the factory organically formed spacial concrete columns are placed as a basis for the pools and set a clear contrast to the existing grid structure. However, the shells emerge from the factory grid as they are constructed and cast with a fabric formwork. Steel cables define the shape of the structure and are covered with geotextile fabric. The fixed steel columns of the factory serve as high points of the steel cable anchoring. They enable the formwork to be built in the factory very efficiently. That makes the geometrically complex structure even possible.