The amount of sea worms used in the film was countless, as the production would order shipments of 250,000 Glycera worms at a time. The production would end up wiping out New England's supply of Glycera fishing worms that year.
The inspiration for the film came from a childhood experiment between director Jeff Lieberman and his brother. One evening the two hooked up a train transformer to wet soil and used the electricity to drive hundreds of worms out of the ground. Young Lieberman noticed that the worms tried to get away from the glare of the flashlight that the boys were using to see by because worms are light-sensitive. It became the scientific basis behind this film and the story of the experiment is re-told by the character of Roger Grimes.
Once in the 1980s, WPIX-TV in New York accidentally showed the film in black-and-white. Instead of complaining, Jeff Lieberman called the station and mentioned how much he loved the way the film looked. In fact, Lieberman prefers people to watch the film in black-and-white even though a black-and-white version is not available. Instead, you should turn the color down all the way on your television set.
The shot of the worms pouring out of the living room ceiling was actually a reverse shot. The camera was turned upside down and filmed live worms being dropped onto a floor that was a mock-up of the ceiling. When the footage was reversed, it appeared as if the worms fell from a hole in the ceiling.
According to director Jeff Lieberman, there was no trickery used in the scene where the tree falls and smashes through the Sanders' dinning room. Lieberman said it was all done in one take with an actual cut tree being released from a crane to fall through the constructed set, complete with the actors on set! Several cameras were placed inside the set to capture the actors literally fleeing for their lives as the large tree landed within feet of them.
The trivia item below may give away important plot points.
For the scene where Willie Grimes is found dead, Carl Dagenhart literally had to be buried in the ground with his head sticking out. A fake body crawling with worms was then joined to his protruding head to give the appearance that his body was just lying on the ground being devoured.