For those of us that are interested in a more technical deep dive below is a a breakdown of the major depth zones and the creatures that live there:

1. Epipelagic Zone (0 - 200 meters / 0 - 656 feet)

• Also known as: The sunlight zone, because this is where sunlight can penetrate.
• Key features: The warmest of all ocean depth zones, most biodiverse zone, supporting a wide variety of marine life.
• Creatures:
  • Fish: Tuna, sharks, sardines, and anchovies.
  • Mammals: Dolphins, whales, and seals.
  • Invertebrates: Jellyfish, squid, and shrimp.
  • Plankton: Phytoplankton (plant-like) and zooplankton (animal-like).
  • Coral reefs thrive here, supporting thousands of species of fish, crustaceans, and other organisms.

At the bottom of the Epipelagic Zone starts the thermocline. In this transitional region, the water temperature decreases rapidly with increasing depth. The depth and temperature of the thermocline vary between seasons and years.

2. Mesopelagic Zone (200 - 1000 meters / 656 - 3280 feet)

• Also known as Mid-water zone and The twilight zone, where light begins to fade, but some faint light from the surface still exists.
• Key features: Light is scarce, and pressure increases as depth increases.  Due to lack of light, it is from this zone that bioluminescence begins to appear from living creatures.
• Creatures:
  • Fish: Lanternfish (known for their bioluminescence), bristle mouth, and hatchetfish.
  • Invertebrates: Squid, some types of jellyfish, and deep-sea shrimp.
  • Adaptations: Many creatures have bioluminescence to attract prey or mates. They also have large eyes to detect faint light.

Despite bursting with aquatic life, this layer has remained relatively untouched from commercial fishing.  Plants do not grow in this zone, so creatures either feed by filtering the water or hunting other creatures.  Humans can dive to this layer but need to wear protective suits due to high pressure and lack of heat.

3. Bathypelagic Zone (1000 - 4000 meters / 3280 - 13,123 feet)

• Also known as: The midnight zone, where no natural light penetrates.
• Key features: Total darkness, high pressure, and near-freezing temperatures.  This zone makes up 90% of the ocean
• Creatures:
  • Fish: Deep-sea fish like the anglerfish, gulper eel, and viperfish.
  • Invertebrates: Giant squid, deep-sea jellyfish, and sea cucumbers.
  • Mammals: Sperm whales are found to dive down this level in search of food.
  • Adaptations: Many animals here are bioluminescent, and have specialized feeding mechanisms to catch scarce prey. Some creatures also have large mouths and extendable stomachs to capture food efficiently.

The pressure in this zone is extreme.  For example 4,000 meters (13,100 feet) down, there is over 5850 pounds per square inch.

4. Abyssopelagic Zone (4000 - 6000 meters / 13,123 - 19,685 feet)

• Also known as: The abyss, this zone is one of the deepest, darkest, most remote areas of the ocean.
• Key features: Constant darkness, near-freezing temperatures, immense pressure, and very little food.
• Creatures:
  • Fish: Abyssal fish like the abyssal grenadiers.
  • Invertebrates: Sea cucumbers, brittle stars, and various types of shrimp.
  • Adaptations: Animals here have adapted to survive with minimal energy, some feed on detritus that falls from the upper layers. Many creatures are slow-moving and have a specialized metabolism.  Most are transparent and blind.

Both the Bathypelagic Zone and the Abyssopelagic Zone are home to Hydrothermal Vents (2000 - 5000 meters / 6562 - 16,404 feet).  These are unique ecosystems powered by the heat from underwater volcanic activity.  Many Creatures are uniquely adapted to thrive in, around, and near hydrothermal vents.  These creatures include: Tube worms, giant clams, and vent crabs, all rely on chemosynthesis (instead of photosynthesis) to produce energy from the chemicals in the vent water, rather than sunlight.

5. Hadalpelagic Zone (6000 meters to the deepest parts of the ocean, about 11,000 meters / 19,685 - 36,089 feet) is the Mariana Trench off Japan in the Pacific Ocean

The regions that exceed roughly 3.5 miles, or 20,000 feet, are known as the hadal zone.  This zone includes the Mariana Trench which is so deep that even Mount Everest would be wholly submerged if placed at its bottom.

• Also known as: The hadal zone, named after the Greek god of the underworld, Hades.  named after Hades, Greek god of the underworld. Fitting right, given their abyss-like nature and mystery?
• Key features: Found in deep ocean trenches and the deepest parts of the ocean. Extreme pressure, cold temperatures, and complete darkness.
• Creatures:
  • Fish: Few species, such as hadal amphipods, and fish found in the deepest trenches (like the snailfish).
  • Invertebrates: Hadel benthic organisms like deep-sea polychaete worms and giant tube worms.
  • Adaptations: Creatures here have evolved to withstand extreme pressures and cold. They often have specialized features, like high resistance to pressure, to survive at these depths.

The Mariana Trench was formed by a process known as subduction, where the Pacific Plate is being subducted under the smaller Mariana Plate.  The Mariana Trench is about 2,550 kilometers (1,580 miles) long and averages 69 kilometers (43 miles) in width. The trench forms a crescent-shaped scar in the Earth's crust.  The deepest diving submarine ever built, the DSV Limiting Factor, can reach a depth of 10,928 meters (36,037 feet).  Few humans have in fact touched the bottom of the ocean. In 1960, the Trieste, a bathyscaphe, descended to a depth of 35,800 feet in the Mariana Trench.

Each of these zones has highly specialized creatures that have adapted to the extreme conditions, from the lack of light to the crushing pressures. The deeper you go, the less food and energy are available, so organisms often evolve ways to survive on little, or have unique features to cope with darkness and cold.