This map marks both a scientific and an imaginative revolution. When it was published in 1977 by Marie Tharp and Bruce Heezen, two oceanographers at Columbia University, there had never before been a map of the entire ocean floor. Here, for the first time, was our planet’s hidden majority. It was as if the plug had been pulled and the water drained out, transforming the blank seascape into a complex landscape of plains and peaks, escarpments and bluffs. Here was the full extent of the Mid-Ocean Ridge, the mightiest mountain range on Earth, its 40,000-mile length making molehills out of more familiar mountains. You can see its serpentine curl through the Atlantic, round the Cape of Good Hope to the Horn of Africa and on through the Southern Ocean, up to 1,000 miles wide and two miles high.

Tharp and Heezen began mapping the individual ocean floors in 1952, but found obstacles in their way. The big one was invisibility: when it comes to mapping the ocean floor, the sea gets in the way of seeing. The second obstacle was limited data. Tharp, who drew the maps, started with the North Atlantic, working from information gleaned on sounding expeditions. Ships travelling across the Atlantic with sonar would fire sound through the sea, working out its depth from the time it took for the echo to be detected. But Tharp had only six complete tracks across the ocean from which to figure out the topography of the whole thing—“six ribbons of light”, as she described them.

As she began analysing the data she noticed something strange, a discovery that would change our understanding of our planet. There was a crack running along the top of the Mid-Atlantic Ridge. Nobody had ever spotted it, but it appeared to be huge—in places wider than the Grand Canyon. Heezen, at first disbelieving, became convinced when it turned out the crack coincided with a string of earthquakes. Tharp’s discovery helped turn a theory then regarded as nonsense into a fundamental fact: continental drift. The sea floor was being pushed up and apart.

“It was a giant paradigm shift,” says Robin Bell, a professor of geophysics at Columbia today. “These mountains now had a reason for being there. That’s the magic place where you in London and I in upstate New York are getting farther apart every day at the rate our fingernails grow. That’s the place where new Earth crust is being made.” It was a leap towards explaining why the Atlantic coasts of South America and Africa fit so snugly together, and why Bermuda is older than the Azores. “It wasn't just random down there,” Bell says. “There was a story.”

And like all good storytellers, Tharp knew to show rather than tell. There had been contour maps of the ocean floors before, showing circles joining points of similar elevation. But Tharp drew physiographic diagrams—maps which show what the sea bed would look like if viewed from above. In her biography of Tharp, “Soundings”, Hali Felt writes that her maps are “a visual argument for the ways the depicted features had been formed”. To make it, Tharp had to extrapolate from the data, using her training as a geologist and mathematician to infer the extent and appearance of those features, to illuminate what lay between the ribbons of light. She has since been proved largely correct.

Tharp—a pugnacious redhead who, in the mostly male world of oceanography, liked to wear long flowing skirts—was as obsessive about her art as her science. In 1961, Felt reports, Tharp visited the printers Williams & Heintz in Washington, DC, who were printing her second map, of the South Atlantic. She thought it had lost detail in the printing, so she picked up a steel point and gouged out valleys, sharpening the definition of the Shag Rocks near the Falkland Islands. “Some of her brilliance”, Bell says, “was that she was able to put it in a way that our eye can understand.”

The maps of the individual oceans were eventually combined into the World Ocean Floor Panorama you see here. There was a scientific point: Tharp and Heezen wanted to demonstrate that we shouldn’t be thinking in terms of separate bodies of water, but of the underlying continuous body of rock. But their purpose was democratic too. They teamed up with the Austrian artist Heinrich Berann, who painted this map using colour to lead the eye—reds for volcanic areas, pale blues for the vast abyssal plains, the Mid-Ocean Ridge purpled like a bruise. “Marie wanted the general public to be able to see the texture of the ocean floor,” Felt writes. It worked on one woman. Robin Bell remembers seeing the map for the first time. “It’s the reason I went into geology,” she says. “I can remember the day. I was like, ‘Oh my gosh, I’m going to be a geophysicist!’”