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Shot back-to-back in Puerto Rico with Last Woman on Earth (1960), using the same cast and crew, but not released until several months afterward.

The scene of the Cuban officers saluting as the little boat they are floating in sinks was not planned; it actually began sinking as they were completing the shot. Roger Corman told the actors to stand and salute as the boat sank and filmed every moment, later stating that it "wasn't very deep" there. He also gave high praise for the Mexican actors who played the officers for taking direction so well.

The little cartoon characters in the opening credits were provided by famed "Mad Magazine" artist Sergio Aragonés, not long before he went to work for the magazine.

The monster was made for $150. The head was made of five helmets from a previous Roger Corman picture. The body was made from a wetsuit, moss, and Brillo pads. The eyes were tennis balls with Ping-Pong ball pupils, and the claws were pipe cleaners. Finally, the monster was covered in black oilcloth to make him slimy.

Renzo's story about a Cuban fisherman by the name of Hemingway is a sly allusion to the fisherman Santiago in Ernest Hemingway's novella "The Old Man and the Sea".