Dipòsit de les Aigües del Parc de la Ciutadella, Water tank and academic library in La Vila Olímpica del Poblenou, Spain.

The Dipòsit de les Aigües is a former water reservoir in the Parc de la Ciutadella, now the Pompeu Fabra University library. It has a quadrangular plan modelled on the Roman Piscina Mirabilis of Naples: parallel load-bearing walls one metre thick, pierced by 14-metre round arches linked by 4-metre barrel vaults, with corner towers topped by galleries and pyramidal roofs.

Built between 1874 and 1880, the structure served as a water tank for the park and was designed by José Fontseré. During his studies, the young Gaudí helped with the structural calculations, making it an early example of his engineering work.

The name Dipòsit de les Aigües recalls the building's role storing water for the Ciutadella waterfall and irrigation. From 1896 to 1977 it housed the Asilo Municipal del Parque, as the inscription over the Wellington Street door still reads. Since 1999 it has been the Pompeu Fabra University library, holding special bibliographic collections.

The building is located near Carrer Ramon Trias Fargas and is easily accessible by public transportation. Visitors should note that the interior spaces are well-lit and accessible during library hours.

The reading mezzanine, raised 2.8 m above the floor, was added by Lluís Clotet and Ignacio Paricio in the 1990s rehabilitation, not part of the 19th-century tank. It doubles the floor area except for a wide central opening. The architects stripped partitions, false ceilings and cladding to leave the brick exposed, and opened a skylight through the vaults that rises above the artificial lake like an island.