In the narrow strait between Jura and Scarba, the sea does strange things. Standing waves barrel over phantom surf breaks. Steely waters seethe and swirl, as if stirred by invisible hands. No wonder the gulf’s name, Corryvreckan, means “cauldron of the speckled seas” in Gaelic; this is the world’s third largest whirlpool, classified as “unnavigable” by the Royal Navy.