This weekend marks the 47th Earth Day, an annual celebration of our pale blue dot. As we reflect on the ground beneath our feet and the flora and fauna whose lives are entwined with ours, it’s worth giving equal consideration to the vast oceans just beyond our shores. The high seas are as vital to life on Earth as terra firma, and with their hugeness and depth, they provide that much more room for myths and mysteries. Here are five of the most persistent.

Myth No. 1

Coastlines are clear boundaries between land and sea.

In a book on landforms and disasters, two scientists call shorelines the “most conspicuous boundary on Earth.” Writing about Toronto’s relationship with the sea, author M. Jane Fairburn notes that the name of the shore town Scarborough is apt, as the word “scar” is related to the Saxon word “sciran,” meaning “to divide” — and “the shore divides the land from the water.” This is certainly how maps are drawn, too: The land ends and the ocean begins.

But the distinction isn’t really that clear. Every few seconds, waves move the edge of the sea in and out. In six-hour cycles, the pull of the moon shifts the level of the water. The combination of waves and tides means the edge of the sea is dynamic, depending on the moon, the wind and the steepness of the shore, which itself changes over time due to erosion.

This movement creates an area of mingled land and sea where certain species thrive in a half-shore, half-ocean zone. Mangroves are some of the only trees that can grow in sea water, some by secreting salt crystals through their leaves to rid themselves of saline. They expand from soil into the shallows as they grow, trapping sediment and creating new land. Marsh grasses do the same, helped by animals that live among their roots and fertilize them. Corals are probably the most stubborn of this group of in-betweeners: They live around mountainous islands, and as the mountains sink under the sea, the corals keep growing toward the surface. Eventually, the mountains disappear, leaving only a ring of coral called an atoll: neither land nor sea, but a space in between.

Myth No. 2

The deep sea is totally dark.

“It’s hard to imagine a more forbidding place than the icy cold, pitch black, crushing environment of the deep ocean,” according to the Smithsonian’s National Museum of Natural History. The New York Times has similarly described the deep sea as “frigid and lightless.”

It’s true that daylight penetrates the oceans only weakly: Below about 3,200 feet deep, there is virtually no sunlight. But there is other light: a starry flickering of blue and green luminescence thanks to an enzyme called luciferase that generates light by breaking down high-energy molecules and producing photons instead of metabolic energy in the bodies of bacteria and some deep-sea fish.

This glittering abyss was captured by the first person to ever travel there: William Beebe, a pioneer of the deep-sea bathysphere . As he sat in his tiny globe in the night of the sea, what stuck with him was not any of the fantastic predatory fish he saw. It was the lights. “We were the first living men to look out at the strange illumination,” Beebe wrote in his book “Half Mile Down .” “And it was stranger than any imagination could have conceived.”

Myth No. 3

Oceans are big enough to dilute pollutants safely.

According to one high school textbook, “Oceans can dilute, disperse, and degrade large amounts of raw sewage and other types of degradable pollutants,” making them handy dumping grounds for human waste. As a handbook on environmental health put it, describing an old way of thinking, “The phrase ‘the solution to pollution is dilution’ epitomizes the common pollution management strategy used over the past centuries, based on the assumption that human actions could not significantly impact such a large diluent as the ocean.”

That was probably true long ago, when the vastness of the ocean could not be harmed by any of the trickling products of scattered human villages. But today, the scale of human industry is so great that even the oceans cannot contain its waste safely.

Over the past two centuries, the concentration of mercury in the top 300 feet of the oceans has tripled because of human activity, including the burning of coal. Likewise, carbon dioxide levels in the air have gone up about 25 percent since 1958 . A great deal of this extra CO2 has dissolved in the oceans, where carbon levels have also increased. Carbon dioxide dissolves in water to create carbonic acid, and an excess of CO2 is leading to higher acidity across the world’s oceans. Despite the vastness of the sea, these pollutants are affecting marine life and ocean ecosystems, dissolving the shells of some creatures and leading to higher mercury content in some species of fish.

Myth No. 4

The hottest parts of the oceans are in the tropics.

If you go looking for the warmest waters around, many sources will probably point you toward the tropics. Climate.gov, for instance, states that “the tropical Pacific and Indian Oceans have some of the warmest . . . temperatures in the world,” and the news site KQED Science notes that “the tropical western Pacific, south of Guam, is the opposite of Antarctica: the warmest part of the ocean.”

But those sites are talking about surface temperatures. The warmest waters in the sea are much deeper, and not necessarily in tropical areas. Far below the waves in places where the planet’s crust runs thin, fiery magma courses just beneath the ocean floor, and burning-hot water screams through underground channels like steam through an old radiator. In the water, superheated minerals mingle and bind into exotic sulfurous compounds. This brew bubbles through small gaps in the ocean floor and can emerge at over 600 degrees Fahrenheit. That’s impossible at the surface; water in your kitchen boils into steam at a little over 200 degrees. But water behaves differently at the base of the world. Its boiling point increases with pressure, so that even 600 degrees can’t induce a boil. The result is, instead, a plume of poisonous fluid so hot it practically glows.

Myth No. 5

Oceans are experiencing unprecedented changes.

Sylvia Earle and Linda Glover preface their book “Ocean: An Illustrated Atlas” by noting that “the ocean is undergoing unprecedented change,” including rising sea levels, declines in marine life and shifting temperatures. Likewise, Duke University’s Nicholas Institute has warned of “unprecedented changes in ocean ecosystems.”

But massive sea changes are not new. Over the past 6,000 to 18,000 years, sea levels have has risen more than 300 feet, drowning shorelines that once were high and dry. Ancient seas have disappeared and been reborn: The Mediterranean, for example, dried up about 5 million years ago, but one of the planet’s major spectacles, the Zanclean flood, refilled the entire sea in as little as two years.

Marine life, too, has vastly changed. Once, the chief predators in the sea were giant cephalopods related to squids and octopi: Some had coiled shells as wide as a human is tall. Then, 400 million years ago, sharks evolved their trademark teeth and have used them effectively ever since to avoid mass extinctions. Some of the newest major animal groups are the most spectacular: Modern baleen whales, the largest creatures ever to live on Earth, evolved just 25 million to 35 million years ago.

What sets today’s transformations apart is not their size or the ocean’s stability until now, but the culprit (human beings), the number and the rate at which these changes are taking place. As Earle and Glover rightly point out, human activity is now altering the oceans at a pace that far outstrips what they have adapted to before.

outlook@washpost.com

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