There are spaces that arise from a lucky and unexpected find. A serendipity of axes X and Z. Faced with the surprise of finding an unforeseen space, the need arises to give it a use and a social dimension thanks to new activities. A space of potential energy that, when discovered and becomes part of our collective knowledge, turns into kinetic energy. A kinetic energy that transforms into social when the space is able to host a program. One find was what happened to the Rei Martí tank, an old water retention tank located on the old estate of the Bellesguard tower and viaduct, the work of Antoni Gaudí. A buried tank of more than 600m2, dated at the end of the 19th century, that had been forgotten under a pine forest until it was accidentally rediscovered 20 years ago. Serendipity demands humility, it requires the ability to fine-tune mechanisms to listen, to connect spaces and people, often far removed from our familiar and reliable environment. This was our first strategy, to maintain this mysterious condition of the deposit and invite the public to reappropriate it and share the excitement of the unexpected find. A small budget allocation allowed the district of Sarrià - Sant Gervasi in Barcelona to project a strategy for the re-appropriation of the deposit to pass its potential energy to kinetic energy and prepare it for future social energy. A city recycling exercise that transforms a water retention tank into a cultural facility. A new space that can be everything. That is why the project space must respect the character, its code, its nature, its materiality, the footprint of the water and the strength of the original deposit. It must be a space that understands and reinforces its structure of pillars, arches every 3.5m and gives value to its Catalan-style ceramic vaults. We could not afford to change the space, on the contrary, we had to promote its strength and its original condition. Access to the tank is via Bellesguard Street, creating a new public space for the neighbourhood that doubles as an exterior entrance hall. A square that saves the unevenness between the street and the reservoir by creating connections and visual relationships with the Bellesguard tower through the materiality of the large concrete walls that integrate with the pine forest above the reservoir. This square also functions as an urban activator with the inclusion of a cafe that appropriates the square and acts as a hinge between the entrance to the tank and the public garden above it. Inside the tank, the ceramic arches and vaults maintain their original character and the imprint of water and lime. Constructive elements that are emphasised with lighting to make them the only protagonists. Meanwhile, the floor and perimeter walls are clad in wood that connects us organoleptically with the forest that is on the roof of the tank and allows us to control the space acoustically. The deposit wants to offer the city the opportunity a different cultural experience.