The Mysterious Ocean
Ask an Ocean Explorer

The Mysterious Ocean

Dr. Jon Copley provides an illuminating tour through the sunlit surfaces and Stygian darkness of the world’s oceans.

Ocean explorers have been studying the deep since the 1800s, yet much of what’s below the surface remains a mystery. Dr. Jon Copley – scientific adviser for the BBC TV series, Blue Planet II – outlines historical, biological and environmental facts and provides glimpses of the ocean’s wonder. His short but surprisingly comprehensive book will delight science nerds and armchair adventurers alike.

Six Miles Deep

The deepest part of the ocean is the Pacific’s Mariana Trench, at 10,916 meters – more than six miles – deep. On average, the Earth’s oceans are 3,400 meters deep. Much of the ocean’s floor remains a mystery.

Nobody tried to depict the topography of the ocean floor until the mid-1800s. Matthew Fontaine Maury charted a portion of the “mid-ocean ridge” formed by volcanic activity in the Atlantic. Engineers laid the first transatlantic telegraph cable atop a vast plateau there.

Rather than sending out just one ‘ping’ at a time to measure the depth immediately under the ship, today’s systems send out a burst of ‘pings,’ which spread out like a fan and simultaneously map a much broader strip of seabed.Jon Copley

Scientists and governments continuously update seafloor maps. Closer measurements enable explorers to find shipwrecks or airplane debris.

Early Underwater Vehicles

Scientists today explore the ocean floor with “Human-Occupied Vehicles” (HOVs), mini-submarines that carry people to the deepest realms, and “Remotely Operated Vehicles” (ROVs), which are less expensive and accomplish much of the same work.

Hot-air balloonist Auguste Piccard designed the “bathyscaph,” an underwater vehicle that adapted a balloon’s hull to withstand underwater pressure. The vehicle carried a tank of gas, which, being lighter than water, created buoyancy. Iron shot acted as ballast, which brought the craft to the sea’s floor, and upon release, returned it to the surface. Belgium funded Piccard’s efforts, and his son Jacques helped him develop and pilot the vehicle.

In November 1948 they collaborated on the first few dives made by the French Navy’s Undersea Research Group, which Jacques Cousteau established. The Piccards developed the bathyscaph Trieste; the US Navy bought it in 1958. In 1960, the Trieste was the first HOV to descend the six miles to the deepest part of the Mariana Trench, Challenger Deep.

Submersibles have successfully completed thousands of missions on the ocean floor, but ROVs cost less and carry fewer risks. A cable connects them to a ship as operators view video and guide the craft. But only HOVs offer the “wide-angle and three-dimensional view human eyes give us.”


The top 200 meters of the ocean are home to algae called phytoplankton floats that utilize the sun’s energy for photosynthesis, forming the basis for the marine food chain.

In fact, around half the oxygen in our atmosphere – or every second breath we take – comes from those dwellers of the sunlit upper ocean.Jon Copley

Under 200 meters, the “dysphotic” zone begins. Most animals in the twilight zone produce their own light through bioluminescence.If a submersible flashes a light, these animals may flash back in response. Most sea creatures live in the sunless mid-water, “aphotic” zone.

The Ocean Floor

Oceanographer Marie Tharp collated echo soundings collected by various ships to map the ocean floor after World War II and posited a valley along the Mid-Atlantic Ridge. Tharp and Bruce Heezen went on to show this underwater rift circles the ocean floor. This discovery confirmed the geological theory of plate tectonics.

The plates of the Earth’s solid crust ride on convection currents in the fluid mantle beneath them, and the mid-ocean ridge lies where those plates are being pulled apart, with molten rock extruding to create new sea floor.Jon Copley

The Cayman Trough in the Caribbean holds five-kilometer-deep hydrothermal vents that gush hot water at higher temperatures than molten lead. Right next to these scorching vents is a rich ecosystem of deep-sea life, including translucent shrimp and anemones feeding on bacteria that grow around the hot springs.

Summer on the Antarctic seabed is rich in life due to 24 hours of sunlight promoting phytoplankton, which feed krill, which feed whales and penguins. On the ocean floor, krill poop nourishes sponges, feather stars and various crustaceans.

Weird Creatures

Perhaps one of the weirdest is a benthic siphonophore, which has a “shaggy barrel” for a body, a tall neck with a float at the top that looks a little like a head. As with most deep-dwelling marine animals, they don’t have to worry about an internal versus external pressure differential because they have no compressible body parts.

Scientists estimate that only around 10% of species living in the oceans have so far been described and given scientific names.Jon Copley

Whales, as air-breathing mammals, face some of the same challenges humans face when diving deep, and are vulnerable to decompression problems, notably when surfacing too quickly when fleeing loud underwater noises.

Mating can be difficult at the bottom of the ocean, in the dark. Octopuses on the ocean floor latch onto whatever other member of their species they come across, whether male or female, a strategy that works roughly 50% of the time.

Human Activity

When you talk on your phone or use the internet, you utilize the fiber-optic cable network on the bottom of the ocean that causes little disruption. By contrast, deep-sea fishing and overfishing decimate populations.

Bottom trawling destroys the entire habitat at the ocean floor, killing coral that need centuries to regrow.

Healthy Oceans

Around the world, nations are designating Marine Protected Areas to help threatened species recover. Governments collaborate to protect endangered species, reduce plastic and other pollution, and tackle climate change.

Why we explore the ocean, and what we do with the knowledge that we gain, defines its future and our own.Jon Copley

Individual, independent actions matter. If people choose inaction, those clinging to the past will impose choices detrimental to the oceans and all animal life.

Into the Depths

Copley writes for adults, though children can understand and gain much from his brief, punchy sentences and refusal to overcomplicate anything. His question-and-answer format actually works, compartmentalizing information and letting you choose what you want to learn and in what order. His expertise is profound, and he’s a lively, readable writer. Think of this delightful book as a gateway to aspects of the ocean about which you knew nothing. Then, after reading it, you will rush elsewhere to explore them.

Other books presenting the ocean’s mysteries include The Brilliant Abyss by Helen Scales; Below the Edge of Darkness by Edith Widder; Why Fish Don’t Exist by Lulu Miller; and, of course, Twenty Thousand Leagues Under the Sea by Jules Verne.

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