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Between 1911 and 1914 reservoir No 5, of 9,091 cubic metres capacity, was excavated from the rock by means of enlarging a tunnel driven parallel to the channel tunnel and some 7.6 metres below it. The tunnel was enlarged on either side to form a chamber some 12.2 metres high 13.1 metres wide and 121.9 metres in length. After the excavation works were completed and all loose rocks removed, the floor was concreted in two layers, a 150 mm levelling layer of mass concrete followed by a 150 mm slab of 1:2:4 concrete using Portland cement and 35mm stone as coarse aggregate. The walls were constructed using two skins each of 114 mm space between the back wall and the rock face was filled with mass concrete and the 150 mm cavity filled with a Portland cement, 1:1 mortar mixed with 5% water proofing agent. The floor received a 50 mm Portland cement, 1:1 mortar screed and finally rendered with a 19 mm layer containing 5% of water proofing agent. The walls were cement plastered in 3 coats using a 3:1, 2:1 and 1:1 with 5% water proofing agent mix. The last layer of wall plaster and floor render was steel trowelled. Nothing was done to the roof as the rock over the span used is self-supporting. The catchment area was increased by a further 5.66 hectares and, in 1928, a further reservoir (No 6) was constructed similar to reservoir No 5, but of half its length and with a capacity of 4,545 cubic metres. Each reservoir is connected to the main channel by means of large diameter pipes with valves to control the entry of water into them. They are now also connected to the pumping mains conveying water from other sources. Between 1928 and 1945 four more reservoirs (Nos 7 to 10) were excavated from the rock adding a further 18,181 cubic metres to the storage capacity. It is interesting to note that reservoir No 10, which had been excavated but not completed at the time of World War II, contained a barracks built to house a detachment of the "Black Watch" regiment under bombproof conditions. The construction of these reservoirs follow the original pattern except that they are offset from the pilot tunnel thus providing access to their supply mains. Outlet valves for supply and cleansing are operated from within the reservoirs by means of long spindled hand wheels. Each reservoir has an overflow onto the pilot tunnel and via the tunnel into the natural fissure. The last reservoirs (Nos 11 and 12) were constructed 1958-1961 at a lower level, reached from the east side, opposite Catalan Bay. These were constructed to receive rain from a further extension of 4.05 hectares of catchment at a lower level on the east side slopes. These brought up the total number of potable water reservoirs to 12 plus the Moorish Castle reservoir providing a total storage capacity of some 72,727 cubic metres. Following the ceasing of operations, of the catchments as a source of potable water in October 1993 the reservoirs are used as service reservoirs and as storage reservoirs providing substantial water reserves.
