A skyscraper one mile high, a dome covering most of downtown Manhattan, a triumphal arch in the form of an elephant: some of the most exciting buildings in the history of architecture are the ones that never got built. These are the projects in which architects took materials to the limits, explored challenging new ideas, defied conventions, and pointed the way towards the future. Some of them are architectural masterpieces, some simply delightful flights of fancy. It was not usually poor design that stymied them - politics, inadequate funding, or a client who chose a `safe' option rather than a daring vision were all things that could stop a project leaving the drawing board.