Plot

The deep sea, which gets darker with increasing depth until no more sunlight penetrates at about a kilometer depth, and ever colder closer to the bottom of the ocean, covers most of the planet and is thus by far the largest habitat on earth, yet has been explored less than space, so most scientific expeditions, at depths requiring modern submarine technology, discover at least one new species, or even whole new branches of submarine life. Like everywhere else, evolution has over millions of years produced several amazing adaptations to even the most extreme conditions, here especially to the lack of sunlight, with its problems for procreation, searching food and fleeing hunters, such as photophore cells which produce specific light types, mimic rare light penetrating from above, etcetera.—KGF Vissers

This episode explores the unknown depths of the ocean. Over 60% of the sea is more than a mile deep and it forms the planet's most mysterious habitat.

A sperm whale descends 1,000 meters to look for food and is followed, with the Johnson Sealink acrylic sphere submersible with a depth range of 900 meters. At that depth, the lungs of the whale are crushed to 1% of their surface volume. At 300 meters it is already very dark, and temperature is dropping fast. On the way down, a number of unusual creatures are witnessed, such as transparent squid and jellies, whose Photophores give pulsating displays of color. Creatures are transparent to avoid being seen and have huge eyes so they can see others. Here creatures have interesting evolutionary adaptations to catch prey in the dark. Some have huge tentacles; others have a web like structure attached to the body that helps catch any passing food.

At 500 meters is the twilight zone. In such dark places, both being able to see (or sense movement) and the means of quick concealment are equally desirable. To that end, some use bio-luminescence as a means of detecting food or evading predators. Hatchet fish have silvery sides so they cannot be seen from the side. They have Photophore cells at the bottom that can mirror any shade of light coming from the top, so they are not visible from the bottom either. Predators have special eyes that distinguish between natural light and light from the Photophores. This is an evolutionary warfare being waged for millions of years.

At 1000 meters, temperature is 4 degrees, pressure is 100 times that at the surface. Life is sparse and hence predators have teeth to deal with food of any size. Predators have organs that help them detect tiny movements in the water, to catch prey. Hairy Anglerfish is a motionless predator and has hundreds of antennae on the body to detect any food that comes near it. The gulper meal has an enormous mouth and a meter long tail. It can consume a meal of any size. Animals create light with Bioluminescence. Animals use the light to attract a mate, or release chemicals into the water. Animals have light bulbs to search for food as well. Most lights are blue in color, with some animals emitting a red color light. Some animals eject a bio-luminescent glue that sticks to their predator and makes them vulnerable to their own predators. Other animals eject bio-luminescent flashes in the water to confuse their predators.

Light only has an effect in the first 100 meters of the ocean and this is where photosynthesis can take place. Below that predators have to look or hunt for food. A descent to the very bottom of the ocean - some 4,000 meters - reveals life even at such cold temperatures, much of it new to science. It is dominated by Echinoderms (sea-cucumbers and sea-urchins) that sweep the seabed; however, there are occasional large hunters, such as Chimaera (which looks like a shark). At this depth, temperatures go below 4 degrees centigrade, and pressure is 400 times that at the surface. Animals stay together as it is difficult to find a mate in the darkness. Corrals at this depth have larger tentacles to capture every particle of food that passes by them.

In addition, Sixgill sharks can grow up to eight meters in length and have remained unchanged for 150 million years. They are described as "living fossils" and relatively little is known about them.

The remains of a gray whale are filmed being consumed by hag-fish, a sleeper shark, probably a Greenland sleeper or a Pacific sleeper and the submersible involved in filming this is DSV Alvin, which is the same submersible that Robert Ballard used in 1986 to explore the wreck of the Titanic, as acrylic sphere submersibles cannot reach that depth.

As the continental slope flattens out, it joins the abyssal plain, which can form huge trenches. At seven miles, the deepest is the Mariana trench, and fish have been found there right down to the very bottom. Only 4 submersibles are available in the world, which have explored less than 1% of the abyssal plain.

Attenborough remarks that more is known about the surface of the Moon. Species captured on film for the first time include the Dumbo octopus and the hairy Anglerfish. The abyssal plains contain the mid ocean ridges, the largest geological structures on the planet. This is the largest mountain chain on Earth and extends for 28,000 miles. Ridges are formed by volcanic lava that solidified. Chimneys still gush hot water which equals the temperature of molten lead. Water remains liquid even at 400 degrees Celsius. Even in these hostile conditions, life flourished. This life derived no energy from the sun. They derive energy from the Sulfides that are pouring from the under-water vents. Ecosystems move as individual vents are rarely active for more than a few decades.

They even found under-water lakes with their own shoreline and sandy beaches. The lake was made of a thick soup of salty brine, far heavier than the surrounding seawater. The sand was made of mussels who lived on the methane gas emanating from the seabed.