Have you ever seen a green flash? It is said to happen as the sun dips below the horizon, creating an elusive green light that appears in the sky for a fleeting moment and then disappears. Many people believe the green flash is a myth, repeated for the benefit of tourists. Others swear by its existence. So is it real? If so, what causes it? Under the Sun reporter Sammy Mack – a skeptic when it comes to the green flash – headed to Key West to investigate this phenomenon.