“AN IRRESISTIBLE MIX OF ART AND GENITALS:” How a classy, A-list historical film turned into a $17 million hardcore porno.

“Caligula,” one of cinema’s oddest and most tawdry experiments, was produced by Penthouse magazine founder Bob Guccione, the massively wealthy publisher who poured his own money into the project about the infamously depraved ancient Roman emperor.

Opposed to the serious drama that director Tinto Brass and screenwriter Gore Vidal desired to make, Guccione commandeered the production in post and added graphic scenes of hardcore sex to spice things up.

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A lot of sex. Multiple, unsimulated orgy scenes leave nothing to the imagination — and go well beyond what most imaginations can whip up. For instance, at Emperor Tiberius’ (O’Toole) home on the island of Capri, a completely nude woman wields a live eel.

Penthouse even published a special collectors edition in January 1981 called “Girls of Caligula,” the same of which could not be said of “Cleopatra” or “Spartacus.”

Critics were appalled at the gratuitous filth. Roger Ebert, generally open-minded, called it “sickening, utterly worthless, shameful trash.”

But starting Friday, a new version of that trash called “The Ultimate Cut” will begin playing select theaters.

Malcolm McDowell and Helen Mirren must be so proud this week.