Proposed art projects that never happened

Proposal for the Princess Diana Memorial Fountain by Julian Opie. Opie's design lost out in the competition to design a memorial for the Princess of Wales to US landscape artist Kathryn Gustafson's shallow rill flowing in a circular stone basin Photograph: Courtesy the artist, Leeds Art Gallery (Henry Moore Institute Archive)

The Ash and Silk Wall by Vong Phaophanit, installed in a park at the Thames Barrier in 1993 but eventually closed due to vandalism Photograph: Courtesy the artist, Leeds Art Gallery (Henry Moore Institute Archive)

Proposal for monuments at King's Cross by the American artist Buster Simpson, which would have used crushed rubble from the site to create colourful terrazzo sculptures Photograph: PR

Balloons lift a sheet to unveil The Horse, inspired by Leonardo da Vinci's never-realised project, in Milan, Italy. Da Vinci spent years designing a giant bronze equestrian portrait of the Duke of Milan, promising the duke's son it would be 'the greatest statue in Italy', but never completed it Photograph: Antonio Calanni/AP

A scale model of Sir Edwin Lutyens' proposed Roman Catholic cathedral, designed in 1931, which would have dominated Liverpool's city skyline but which was never built Photograph: Christopher Thomond/Christopher Thomond

Calton Hill in Edinburgh, Scotland. The columns were to be part of a grand building commemorating the Scots who died in the Napoleonic wars, but the project was abandoned when the money ran out Photograph: Jeff J Mitchell/Getty Images

Marc Brunel watercolour of the proposed Isambard Kingdom Brunel 1843 tunnel under the Thames in London, planned as a dual carriageway for horse-drawn vehicles Photograph: Brunel Exhibition Rotherhithe