## TL;DR

Oceans cover 71% of Earth's surface and contain 97% of its water. Average depth: 3,688m. Zones: sunlight (0-200m, photosynthesis), twilight (200-1,000m), midnight (1,000-4,000m), abyssal, hadal (trenches, >6,000m). Deep ocean: largest ecosystem on Earth, largely unexplored. Marine life ranges from microscopic plankton to blue whales (largest animal ever, 30m, 200 tons).

## Core Explanation

Phytoplankton: produce 50-80% of Earth's oxygen. Coral reefs: biodiversity hotspots (25% of marine species in <0.1% of ocean). Bioluminescence: 90% of deep-sea organisms produce light. Pressure: increases 1 atmosphere per 10m depth. Hydrothermal vents (1977 discovery): life powered by chemosynthesis (not sunlight). Top predator: orca (killer whale). Marine snow: organic material falling from surface — deep sea food source.

## Further Reading

- [The Biology of the Deep Ocean (Peter Herring)](https://global.oup.com/academic/product/the-biology-of-the-deep-ocean-9780198549567)