"What Lies Below" is the 12th episode of the second season of the American science fiction drama television series Fringe. Set in a quarantined Boston office building, the episode revolves around a thousands-year-old viral hemorrhagic fever that infects and then influences its victims to attempt to spread the viral particles outside of the building. "What Lies Below" first aired in the United States on January 21, 2010, on the Fox network. An estimated 6.90 million viewers watched the episode, and it received mixed reviews from critics, with one calling it average and predictable.