As Ruffnut and Tuffnut succeed at building a tower (or two or three), Fishlegs, studying the Dragon Eye, discovers the location of legendary island Dark Deep, the nearly mythical homeland of Gronkles (Meatlug's dragon species). Flying there with Hiccup, they discover resident Gronkles cowed under the formidable presence of a large, tumbling dragon. Classifying it as a Catastrophic Quaken, Hiccup's all for leaving the situation alone and letting nature take its course, but Fishlegs enlists Snotlout's aid in using dirty tactics to get the Quaken off the island instead.
—statmanjeff