The valorization of a historical artefact is a subtle operation which has to manage to understand the historical potential and to translate it in a powerful and contemporary conformation. The Borbonic water tank of Formia is underneath the “Guglielmo Marconi” square in a public space with value near the sea and the docks. The first reflection that was done, was about making this space be perceivable in the above surface. The contemporaneity of the double is a key element of such project. The double technical value of the architectural intervention, which brings light in the dark spaces of the water tank and where the chimney sucks fresh air of the water tank in the public space creating a distortion of the eye sight and at the same time tries to fade away lo leave space to the city. The double direction of the project, horizontal by joining the public space with the city and the city becoming its center, and vertical in connecting the two worlds, the visible one of the piazza and the hidden one of the water tank. The project so tries and realize the complexity of these actions with a simple and recognizable gesture, the design of a wall, a cut in the horizontal space of the piazza, and in the verticality of the perceived public void. Designing a project which both has some qualities contributing to the environmental re-qualification of the space and that works on several levels is at the same time like a virtual joint. To do so parallel reflections have been carried on, one in the surface regarding the connection of the public space with the built context and the underground one with the volume of the water tank.