The southern Line Islands sit in one of the most remote stretches of the South Pacific, and marine ecologist Enric Sala leads a National Geographic team more than 3,300 kilometers across open water to survey them, reef by reef, from microbes to sharks. On Flint Island they count over three times the coral cover found on any other reef in the Indo-Pacific. Malden Island, used as a nuclear test site in 1958, now holds ten times more sharks than any other reef the team has studied. Millennium Atoll turns out to be a refuge for blacktip reef sharks, and Starbuck Island has the second-largest fish biomass ever recorded on a reef. The footage moves between underwater surveys, shark counts, and coral transects as the scientists build a picture of what an undisturbed reef actually looks like, an ecosystem with almost no human pressure to compare against the degraded reefs most research is based on. The film uses these islands as a baseline, a rare chance to see reef health without the usual math of overfishing and pollution already factored in.