The photic (“sunlight”) zone is the only place in the ocean where there’s enough light to support photosynthesis. Although it makes up only two per cent of the ocean, it houses around 90 per cent of its known life. Fish, seals and crustaceans all call it home. Sea algae, which makes up 98 per cent of the biomass in the ocean and can grow nowhere else but in the photic zone, is essential to all life. Seventy per cent of the oxygen on Earth comes from ocean algae. Without it, we couldn’t breathe. How algae can generate so much oxygen and how that might be affected by climate change, nobody knows. That’s part of what the Aquarius aquanauts are trying to find out. They’re also trying to crack more mystical marine riddles, such as the secret behind coral’s “telepathic” communication.