Interesting facts according to IMDB:
-During the hallucination sequence in the cave, the pyrotechnic charges underneath the mushroom-shaped rock are visible as it elevates, as is the wire lifting it up.
-Author Paddy Chayefsky disowned this movie.
-Arthur Penn was originally slated to direct but resigned.
-The book was partially based on dolphin researcher John Lilly, who invented the isolation tank, and first started taking drugs while "tanking".
-Film debuts of both William Hurt and Drew Barrymore.