No film can ever hold as much intrigue as the cancelled project, that fabled, elusive "what if?" built up in press events and promo images, only to fall apart and become part of cinema's historical refuse pile. And there is no genre more given to this phenomenon than science fiction. There are sci-fi movies that were discarded by studios decades ago that still manage to cling on to an undead sort of cultural relevance -- a fascination among fans that won't let them truly disappear.

Today, we're looking at some of the most overhyped, widely discussed, heavily promoted, but ultimately never-to-be sci-fi films. It's a collection that undeniably sparks imagination, making us wonder what might have been had studio money lined up in the right way, or rights moved into the proper hands a little sooner, or a big star volunteered to join up.

We'll never know exactly what these movies would have looked like,...