For centuries, mariners spun tales of gargantuan waves in the open ocean, annihilating walls of water measuring 100 feet high or taller. Though once dismissed as impossible, new evidence has led oceanographers to determine that we have underestimated how the seas behave at their most ferocious. Journalist and author Susan Casey provides a look at these colossal, ship-swallowing rogue waves—noting that as scientists scramble to understand this phenomenon, others, including extreme surfers, view the giant waves as the ultimate challenge.