Posidonia (Posidonia oceanica) is not a seaweed but watery flowering plant, endemic to the Mediterranean sea. It is everywhere, representing 25% of the seabed. They form an ecosystem called the ‘posidonia meadow’. Of the 600 species of fishes in the Mediterranean sea, 20% live there. More than half of the oxygen we breathe comes from seas and oceans, and posidonia is a true oxygen tank. Posidonia meadows are often compared with the Amazon forest. Posidonia is one of the five protected species in the Mediterranean sea.