Where Is the Deep Sea? Below 200 Meters, Where Sunlight Dies

The deep sea begins at the continental shelf break (~200 m depth) and extends to the ocean floor (avg 3,700 m; max 11,000 m). It covers 90% of ocean volume and 65% of Earth's surface.

Depth Zones (Pelagic)

Benthic Realms (Seafloor)

• Continental slope: 200–3,000 m, steep, canyons, cold seeps, coral mounds.
• Abyssal plains: 4,000–6,000 m, flat, sediment-covered, manganese nodules, polymetallic crusts.
• Seamounts: Underwater mountains (100,000+ >1 km tall), biodiversity hotspots.
• Hydrothermal vents: Spreading centers, 300–400°C chemosynthetic ecosystems (tube worms, vent crabs).
• Trenches: Subduction zones, hadal zone. Mariana (10,924 m), Tonga (10,820 m), Philippine (10,540 m).

Why 200 m?

Compensation depth: photosynthesis = respiration. Below, no primary production—all energy imports (marine snow, carcasses, chemosynthesis). Pressure >20 atm. Temperature ~4°C. It's a different planet.

Human Access

• SCUBA: <40 m (rec), <100 m (tech).
• Atmospheric suits: ~300 m.
• Manned submersibles: 5 (Limiting Factor, Alvin, Deepsea Challenger, Jiaolong, Shinkai 6500) — full ocean depth capable.
• ROVs/AUVs: Workhorses (thousands of dives/yr).

The deep sea isn't a place—it's most of the planet's living space. We've seen <0.01% of it.