Boca do Inferno (in English: "Mouth of Hell") is a scenic cliff formation located west of Cascais, Portugal. It gets its name from the rough ocean waves which crash against the cliff face, forcing their way into a cave system, and spraying angrily from an opening above.

The cave has been a tourist attraction for centuries. One of the first "actuality films", A Sea Cave Near Lisbon (1896), was shot from inside the cave to showcase the majestic waves pouring in.

Boca do Inferno is better known as the place where Aleister Crowley, the famed astrologer, magician, and occultist faked his death in 1930. With the help of poet Fernando Pessoa, he was able to give the appearance of a suicide (perhaps inspired by the death of the Greek philosopher Empedocles, who threw himself into a volcano). Pessoa handed Crowley's suicide note to newspapers, helpfully explaining the Thelemic symbols and translating mangled Portuguese to police and media alike. Crowley reappeared three weeks later at the opening of an exhibition of his works in a Berlin gallery, suggesting this whole affair was more publicity stunt than anything else. The content of the note also indicates it was intended to annoy his lover and magical partner. 

Today, there is a small white plaque mounted on the rock commemorating the event. It tells the story of the pseudocide and provides the text of Crowley's suicide note: "Não Posso Viver Sem Ti. A outra 'Boca De Infierno' apanhar-me-á não será tão quente como a tua" which translates roughly to "Can't live without you. The other mouth of hell that will catch me won't be as hot as yours." That might be touching if any of it were genuine.