Edinburgh sells two bestselling ways to get underground after dark, and their listings are nearly indistinguishable. Both mention vaults; both mention ghosts; both promise the dark side of the Old Town. They are, in fact, very different evenings. The short answer: the Underground Vaults Tour is a history tour that happens to be underground; atmospheric, candlelit, and genuinely fascinating, but nobody is trying to make you scream. The Evening Underground Ghost Tour is the theatrical one; a costumed character walks you through a 450-year-old graveyard and then underground, and it wants you nervous. Pick based on whether you want to learn…