Directly above Sandy Bay and the Sir Herbert Miles coast road were Gibraltar's large water catchments, which are no longer in use. The slope on which they sat is the Great Gibraltar Sand Dune, an ancient consolidated sand dune which dominates the east side of the Rock of Gibraltar. The dune's naturally smooth surface allowed for the construction of a water catchment area made up of a corrugated steel sheets and a layer of concrete. Rainwater flowed down the slope into an open channel which fed into the reservoir system inside The Rock.