Ever wondered what a mermaid vagina would look like?

I certainly hadn’t, until I saw “The Lighthouse.” But that’s only the tip of the surrealist iceberg in this beautiful black-and-white head trip from director Robert Eggers (“The Witch”), where Robert Pattinson and Willem Dafoe battle the elements, drink, dance, bellow, fight, fart and stumble toward the edge of madness as lighthouse watchmen stranded on an isolated rocky outcropping. Eggers used the outline of a Welsh true story and the writings of Herman Melville, Robert Louis Stevenson and dictionaries of sailor lingo to create his two-man drama set in the late 1800s.

Dafoe’s character, Thomas Wake, is the old-salt keeper of the light, who torments his newly arrived assistant (Pattinson) with endless backbreaking tasks and berates him for not doing them fast enough. What happened to the old assistant? Pattinson’s Ephraim Winslow can only wonder at Wake’s story about the boy being driven mad by the gleam at the top of the tower. But Wake has his doubts about Winslow’s seemingly placid backstory as well.

The film starts slowly, with occasional blasts of the lighthouse horn and the smash of waves into the rocks and wooden decks. Toots from the flatulent Wake demonstrate the reality of living without any modern ambient noise, and add a nice comic touch to the overarching aura of gloom. Soon, though, the dynamic between the taciturn Winslow and the taskmaster Wake has disintegrated into something far more menacing. By turns funny, sinister, haunting, historically fascinating and mythical, “The Lighthouse” is one of the best films of the year.