The H.P. Lovecraft quote shown just before the credits is a paraphrasing of the last line of "The Shadow Over Innsmouth", the story that most of the movie is based on: "We shall swim out to that brooding reef in the sea and dive down through black abysses to Cyclopean and many columned Y'ha-nthlei, and in that lair of the Deep Ones we shall dwell amidst wonder and glory for ever."
Brian Yuzna was in his car once with his wife, Kathy, and they were stopped by a gunman. Although terrified, Yuzna grabbed the gun and started smacking the guy with it. They ended up fighting, and Kathy started hitting the gunman with her cellphone to get him off Brian. She said "I've got to get a bigger cellphone," hence the use of the line in the film, which the filmmakers felt was representative of heroics of ordinary people.
The name of the cursed town is Inboca or "Imboca" (according to Spanish language rules). Boca means mouth in Spanish, so translating and adding an "S", you get In-s-mouth, the original name of the town created by H.P. Lovecraft for his "Shadow over Innsmouth" story.
When Paul and Bárbara come close to Imboca, both see strange buildings supported over four pillars with a trident symbol on the roof. These buildings are actually raised granaries ("hórreos" in the original Spanish), a typical building in the north of Spain used to keep the harvest collected during the summer and preserve it for autumn and winter. Raised granaries are crowned by the Christian cross, changed for the movie to the symbol of Dagon's cult, a trident. When the movie ended, the tridents were retired.
Dagon appears in the Judeo-Christian Bible and is the name of a half-fish/half-man deity who is the father of Baal. Dagon was worshiped by the Assyrians in Ninevah, whom the prophet Jonah was sent to preach to.
Some of Imboca's inhabitants have inter-digital membranes, gills, and the inability to blink or move rightly. All characteristics mentioned in the original story written by H.P. Lovecraft to describe Innsmouth's population.
Imboca has a church called "Esotérica Orde De Dagon" (Esoteric Order of Dagon) instead of the typical Christian church. Esoteric Order of Dagon is a fictional cult appeared in "The Shadow over Innsmouth" and "The Trial of Cthulhu". The symbol of the order, something similar to a giant eye, was created specially for the movie.
In the original plot Howard, Vicki, Paul and Bárbara travel in the boat celebrating the success of their company "dot.com", named after the term and domain name created in 1995 (which in tern is derived from the word "commercial"). Curiously, when Paul returns to the ship looking for Howard and Vicki, it can be seen that the name of the ship is "RedHen.com".
Another adaptation of "Shadow Over Innsmouth" was to have been produced by Full Moon Entertainment in the early-to-mid 1990s, and was written by Craig Hammond. However, it never went past pre-production.
The trivia items below may give away important plot points.
The fictional Esoteric Order of Dagon inspired the real Esoteric Order of Dagon Amateur Press Association, created in 1973 and formed by writers and studious of the Mythos of Cthulhu and all H.P. Lovecraft's stories.
In "The Shadow Over Innsmouth" the cult of Dagon was lead by the Marsh family. In the movie, the cult of Dagon is lead by the Cambarro family. They were named after Combarro, the town where the movie was entirely filmed.