Silent Valley Reservoir is a reservoir located in the Mourne Mountains near Kilkeel, County Down in Northern Ireland. It supplies most of the water for County Down, surrounding counties and most of Belfast via the Aquarius pipeline. The reservoir was built between 1923 and 1933 by a workforce of over one thousand men, nine of whom died during construction. The catchment area is 9,000-acres (3,600 ha / 36 sq. km).