Plot

Endless blue stretches in every direction. The sea bed is a staggering eight kilometers deeper down and the nearest island is 500 kilometers away. There is nothing save the burning sun above and the blackened abyss below. How, then, does life exist?—Anonymous

The third installment focuses on life in the "marine deserts": seas that are furthest from land. These are open oceans, with no land in sight. Such waters contain the swiftest and most powerful of ocean hunters.

A feeding frenzy is shown, as striped marlin (voracious predators who can grow up to 3 meters long and hunt in the daylight, to a depth of about 100 meters from the surface), tuna and a Sei whale (later identified as a Bryde's whale) (14 meters long, weighs 20 tons and has an appetite to match) pick off a shoal of sardines until all within it have been consumed. Feeding frenzies like this last for a short while before the prey are consumed and the predators have to move on

Manta rays (measuring over 5 meters across from tip of one wing to the other) also gather to eat the eggs of spawning surgeon-fish. The eggs are sprayed by the fish in ocean water and all the Manta have to do is to swallow the water and sift out the eggs.

Eggs of the Yellow fin Tuna take 2 years to mature into adults. They are typically 2 meters long and weigh 200 kilograms. Only 1 in a million live as long as 3 years. As the eggs hatch, the tiny fish are hunted by birds who hover on the ocean's surface and pick the tiny fish as food. The yellow fin tuna baby fish are carried around the ocean by strong currents. Night-time brings new and dangerous predators such as the jelly fish.

Accumulations of plankton correspond to ocean 'boundaries' (where the waters from the 2 oceans meet but cannot mix) and consequently, schools of plankton eating fish seek them out. They hunt for prey like Sardines. This in turn attracts predators (such as the spotted dolphins off the coast of Panama). The dolphins can track their prey using sonar from hundreds of miles away. The prey has few places to hide, and their only defense is to gather into a ball-like structure. Any fish left outside the ball are easily picked off. The noise of the dolphin attack attracts the attention of a sailfish who has detected rapid vibrations in the water and is looking for the cause. A sailfish is filmed on the attack. Sailfish relies on eyesight for the hunt and hence is only active during the day.

A blue shark also surfaces to reheat in the warmer waters, after a prolonged hunting session at a depth of 300 meters. The only escape for smaller fish is to put as much distance between them and their pursuers as possible. The ocean is full of creatures who ride the currents for thousands of miles every year in the hope to avoid predators, find food and find a safe place to breed.

Bluefin tunas are able to heat their bodies and so can hunt in colder conditions than the others of their species. They travel thousands of miles from their spawning grounds in the tropics to hunt in the cold seas.

Creatures include crabs, turtles, manta rays and even flying fish (who lay their eggs on a single piece of flotsam in the ocean). The flotsam is chased by predators for weeks who wait patiently for the eggs to hatch and to feed on the offspring. A single piece of flotsam in the open ocean can support a fish population of hundreds of tons. The flotsam also create symbiotic ecosystems, such as the sun fish who hunts jelly fish in the deep ocean and gets covered with parasites. The sun fish swims to the flotsam to find a particular kind of fish called moon fish that eats all the parasites stuck to its body. The floating sea gulls also help the sun fish to eat the parasites off its body.

Off the coast of New Zealand, an undersea volcano has formed an island, and the nearby currents sweep many kinds of creatures to it, again creating huge feeding grounds. The islands are far from being safe havens for the small fish or the shrimp that get attracted to the plankton growing around the islands fed by nutrients from the volcano. The Pacific Ocean has over 23,000 islands.

Another Pacific seamount (a mountain or a volcano completely submerged in water) is surrounded by hammerhead sharks, but not to seek food: they are there to allow other fish to clean them of parasites. However, others that are on the lookout for prey arrive in vast numbers. Once again, the hunt comes down to a feeding frenzy with the prey trying to defend themselves by grouping together in a ball-like structure. The plankton moves with the current and this forces the predators to move as well.

A large pod of common dolphins is too big to feed all at once and so splits up into smaller expeditions. The timing of the year determines the weather, and the ocean temperature, and this in turn decides the concentration of plankton in the particular ocean area.

One of the dolphin expeditions ends up near the Azores with a shoal of mackerel in its sights, but they have to compete for their quarry with an attendant flock of shear-waters (birds who can dive down into the water to a depth of several meters to hunt for fish) and a group of adult yellow-fin tuna.