At 200 metres below the ocean surface, the last visible light disappears. At 1,000 metres, pressure is one hundred times what it is at sea level. At 4,000 metres — the average depth of the world's oceans — the temperature is approximately 4 degrees Celsius and has been that temperature, continuously, for millions of years. Approximately 95% of the ocean's volume exists below the sunlit zone, and humans have explored less than 20% of it.
Life Without Photosynthesis
The deep ocean violates one of the foundational assumptions of conventional biology: that all life ultimately depends on photosynthesis, on sunlight converted to chemical energy through plants. At hydrothermal vents in the deep ocean floor, entire ecosystems exist on chemosynthesis — bacteria converting hydrogen sulphide from volcanic vents into energy, and larger organisms feeding on those bacteria. This is life with no relationship to the sun whatsoever.
"The deep-sea vent ecosystems are the best analogue we have for life that might exist on Europa or Enceladus. They show us that life doesn't need a star. It needs chemistry and energy. Those exist everywhere."
Why It Matters Now
Deep-sea mining proposals are currently under review by the International Seabed Authority, which regulates resource extraction in international waters. The commercial interest is real: polymetallic nodules on the ocean floor contain cobalt, nickel, and manganese in concentrations that would, at scale, meet global demand for battery materials for decades. The ecological cost of extracting them from ecosystems we have barely begun to understand is impossible to calculate — which is, depending on your perspective, either a reason to proceed carefully or to stop entirely.