They produce twice as much oxygen in the Mediterranean as the Amazon rainforest. They are often confused with algae. But seagrasses, known as "posidonia" in the Mediterranean, are plants that are crucial to our planet. Yet they are under threat. Scientists believe the Mediterranean has lost 14 percent of its seagrass meadows over the past 100 years. Tourism, boats dragging their anchors and pollution have driven the rapid decline of this ancient, carbon-absorbing plant.