 In the sixth-season episode "Across the Sea", young Jacob and his unnamed fraternal twin brother are shown a cave with a waterfall; in "The End," Desmond descends into the cave and discovers an ancient chamber.In the chamber there is a pool of electromagnetic energy with a large hieroglyphic stone that sits at the center covering a mysterious hole.When it is removed by Desmond, the water and energy drain from the pool, an infernal red glow infuses the chamber, and the island begins to violently react.