The Patriarcal Reservoir, also known as the Praça de D. Pedro V Reservoir, is located underground, beneath the Príncipe Real garden. It was designed in 1856 as part of the project to supply water to Lisbon by French engineer Louis-Charles Mary.

The reservoir, built between 1860 and 1864, was designed to supply the downtown area of Lisbon. Its octagon shape matches the polygon represented by the iron fencing around the lake that lies over the water deposit at the centre of the Príncipe Real garden.

The cistern was initially supplied by the Águas Livres Aqueduct and as of 1833 by the Alviela system. It was built in stone masonry with two compartments with total capacity of 884m3 of water. This reservoir’s main job was to regulate the pressure between the Arco Reservoir (on Rua das Amoreiras) and the piping system of the city’s downtown area.

The thirty-one 9.25 m pillars, with different widths, support the stonework arches, which in turn support the cupolas. Over the cupolas lies the basin (lake) with the waterspout. Both the lake and the waterspout were for airing the water before it entered the deposit.

The spouted water entered the reservoir through four openings at the bottom of the basin, which had tubes that went all the way up to the water surface and acted as drains.

Three underground galleries start at this reservoir:

• The first one starts from the wall on the eastern side (3m from the bottom) and will meet up with the Loreto gallery. It transported the water from the Arco Reservoir
• The second gallery is located below the first and used to follow onto Rua da Alegria, where it ended its trajectory
• The third gallery started from the wall on the western side towards Rua de S. Marçal and supplied the western part of Lisbon

The Patriarcal Reservoir was deactivated at the end of the 1940’s. It has been part of the Water Museum since 1994 and the museum promotes and conducts free and guided tours of this space.