Nelsons Column is some 170 foot high, with the statue of Nelson himself being some 18 foot high.
One plinth has been vacant for 158 years, subject of endless proposals and counter proposals for memorials, until Prue Leith, then chairman of the Royal Society of Arts, decided it should be filled by a series of temporary contemporary art installations.
Rachel Whiteread's Monument is an upended mirror image of a 14-ft high granite plinth, cast in two huge sections of clear resin which change colour with every change of light.