What Would We Find If We Drained The Oceans

Off Guam, the seafloor falls away into the Mariana Trench, and the deepest known point on Earth lies inside US waters, its floor protected since 2009 as a national monument. Water covers roughly 71 percent of the planet, but most of what lies beneath is unexamined. NOAA has mapped about 56 percent of the seafloor in US waters to modern standards, yet explorers have directly seen less than a thousandth of a percent of the global deep seafloor, an area about the size of Rhode Island. Draining the water away would expose continental shelves wide enough to walk between continents, a mountain chain circling the globe underwater, plains flatter than anything on land, and trenches deeper than any canyon. Using an animation updated by planetary scientist James O’Donoghue and NOAA’s ETOPO1 global relief model, here is what would be left behind.

The continental shelf is the submerged rim of a continent, ending where the seafloor drops away into the slope. The first ground to break the surface is the continental shelf, the gently sloping rim that fringes every landmass and accounts for about 8 percent of the area covered by the oceans. Geologically, it belongs to the continent it fringes rather than to the ocean basin, which means a continent’s real limit sits out at the shelf break, not at the waterline a map draws. The break arrives at a strikingly consistent depth worldwide, close to 140 meters (460 feet). That consistency is itself a record of the ice ages, when sea level stood roughly that far below its present position and waves worked the shelf edge at that line for thousands of years.

How far out the limit falls varies enormously. Off parts of California, the shelf runs less than a kilometer (0.62 miles) before the seafloor drops away; off the northern coast of Siberia, it reaches roughly 1,290 kilometers (800 miles) from shore. The difference is tectonic. Narrow shelves are found at active margins, where a plate is being consumed a short distance offshore, while broad ones sit on passive margins that have accumulated sediment undisturbed for tens of millions of years. Averaged across the planet, a shelf measures about 65 kilometers (40 miles) across. The polar shelves are the exception that the animation makes obvious: around Antarctica, the shelf averages some 500 meters (1,640 feet) deep, pressed down by the weight of the ice sheet and overdeepened by ice streams grinding seaward, so the polar margins stay flooded long after the temperate ones have dried.

Only the narrow Bering Strait separates Russia from Alaska; during the last glacial period this gap was dry land. Lower sea level by 130 meters (425 feet), the position it held at the height of the last ice age between about 26,500 and 19,000 years ago, and the shallowest shelves become walkable ground. The Bering Strait is the clearest case. Roughly 85 kilometers (53 miles) of water separate Russia from Alaska today, over a sill only about 50 meters (165 feet) deep, so a fraction of that drop is enough to weld the two continents together. The plain it exposed, Beringia, was arid grassland rather than tundra. Horses and mammoths grazed it, and the climate was dry enough that standing lakes were rare and rivers shrank to streams.

Nitrogen-isotope records from Arctic sediments date the last emergence of the land bridge to about 35,700 years ago, much later in the glacial cycle than earlier work assumed, and rising seas closed the strait again between roughly 13,000 and 11,000 years ago. The North Sea tells a slower version of the same story. Doggerland linked eastern Britain with the Netherlands and Jutland, and it drowned by degrees rather than at a stroke. Rising water had already broken it into an archipelago by the time the Storegga Slide, a submarine landslide off Norway, sent a tsunami across the basin about 8,150 years ago. Even then, the highest ground stayed dry for centuries. Not every gap was closed, however. Even at the lowest sea level, deep water still separated the Sunda Shelf of Southeast Asia from the joined landmass of Australia and New Guinea. The first people to reach Australia, at least 50,000 years ago, had to cross the open sea by boat.

Remotely operated underwater vehicles are used to map the abyssal plains and other deep ocean features. Past the shelf break, the seafloor descends in earnest. That drop is the continental slope, which averages about three degrees of gradient and is cut in places by submarine canyons as deep as any river gorge on land. At its base, the angle eases into the continental rise, an apron built by turbidity currents that carry sediment down the slope in pulses, and the rise feeds onto the abyssal plain at depths between 3,000 and 6,000 meters (10,000 and 20,000 feet).

These are the flattest surfaces anywhere on Earth, with elevation changes of less than a meter over a kilometer of horizontal distance, because a sediment drape that can approach a kilometer in thickness buries the volcanic topography beneath. They are not evenly distributed. The Atlantic and Indian oceans carry the largest plains, fed steadily by wide passive margins. The Pacific has few because the trenches around its rim intercept sediment before it can spread across the basin. The Sohm Plain in the North Atlantic alone covers about 900,000 square kilometers (350,000 square miles). Estimates of how much of the ocean floor the plains cover vary widely, between roughly a third and 70 percent, depending on where the boundary is drawn. What interrupts them are the abyssal hills and the volcanic peaks known as seamounts, most of which never reach the surface.

The mid-ocean ridge system winds through every major ocean basin, tracing the boundaries where tectonic plates pull apart. Credit: National Oceanic and Atmospheric Administration, Public domain, via Wikimedia Commons Where two tectonic plates pull apart, mantle rock rises into the gap, melts under falling pressure, and cools into new oceanic crust. That process is seafloor spreading, and the ridge it builds is continuous through every ocean basin. Strung end to end, it measures close to 65,000 kilometers (40,390 miles), the longest mountain chain on the planet, with more than 90 percent of it in deep water and out of sight. The Atlantic Ocean has been widening along its share of that seam since Pangea broke apart.

The Mid-Atlantic Ridge is slowly separating, 2 to 5 centimeters (0.8 to 2 inches) a year, and the thin magma supply allows the axis to collapse into a rift valley comparable in depth and width to the Grand Canyon. The East Pacific Rise moves several times faster, 6 to 16 centimeters (2.4 to 6.3 inches) a year, and rides high and smooth with no central valley at all. The ridges also settled a much larger question. In February 1977, scientists working on the Galapagos Rift in the submersible Alvin found hydrothermal vents surrounded by clams and tubeworms, an ecosystem that lives on chemical energy rather than sunlight. Two years later, the first black smokers turned up on the East Pacific Rise.

Giant Tube Worms Riftia pachyptila thriving at a deep-sea hydrothermal vent in the Mariana Trench. Via Shutterstock / Gallwis The trenches are the opposite of the ridges. Where a ridge marks plates separating, a trench marks one plate bending down beneath another and returning to the mantle, which is why they hug continental margins and island arcs rather than the middle of basins. Once the animation has taken sea level down by about 6,000 meters (20,000 feet), almost everything else is dry, and the lowest ground on the planet still holds water. The Atlantic holds two of them. The Puerto Rico Trench follows the northern edge of the Caribbean, and its Milwaukee Deep reaches 8,380 meters (27,490 feet), the deepest point in that ocean. Further south, the South Sandwich Trench separates South America from Antarctica and reaches a depth of about 8,265 meters (27,100 feet) at the Meteor Deep.

The deepest are all in the Pacific Ocean. The Peru-Chile Trench, also called the Atacama Trench, drops past 8,000 meters (26,000 feet) off South America. The Tonga Trench in the southwest Pacific holds the Horizon Deep at roughly 10,820 meters (35,500 feet), the second deepest point known. The Philippine Trench bottoms out at the Emden Deep, 10,540 meters (34,580 feet). North of those sits the Kuril-Kamchatka Trench, and last to empty is the Mariana Trench, where the Challenger Deep measures about 10,935 meters (35,876 feet). If you set Mount Everest on that floor, more than 2 kilometers (1.3 miles) of water would still cover the summit. Pressure there exceeds a thousand times the load at the surface, which is why the place has been visited so rarely. Jacques Piccard and Don Walsh took the bathyscaphe Trieste to the bottom in 1960, and nobody repeated the dive for 52 years.

Rain begins at sea, since most of the water that evaporates into the atmosphere comes from the ocean surface, so a drained Earth would have no meaningful water cycle, and the continents would harden into deserts. NOAA estimates that at least half of the planet’s oxygen production happens in the ocean, most of it by drifting plankton in the sunlit upper layer; the atmosphere holds a reservoir built over hundreds of millions of years, so breathing would not stop the next morning, but the engine that fills the air would have stopped. The ocean also works as the planet’s thermostat, carrying heat from the tropics toward the poles, and without it, the days would scorch and the nights would freeze. That is what makes the question worth asking: the drained seafloor is a map of a system we depend on completely and have barely begun to look at.