Marie Tharp grew up with mapmaking. Her father was a soil scientist for the Department of Agriculture, surveying land and making maps. The family followed his job across the country, and Marie attended nearly two dozen schools before she graduated from high school in the late 1930s. This experience taught her how to analyze geologic features of the land.

She was a student at the University of Ohio when Pearl Harbor drew the U.S. into World War II. The war emptied colleges of male students, and left many job openings vacant. Tharp, finding that the University of Michigan’s geology department was suddenly open to female students for the first time, earned a master’s in 1943. She learned geologic mapping and worked as a draftsperson for the U.S. Geological Survey. In 1948, she got a job with Columbia University in New York City, eventually assigned to be an assistant to geologist Bruce Heezen, a newly minted Ph.D.

Sonar had recently been invented and Navy ships were outfitted with sonar that could plumb the depths of the sea. At the time, it was thought that sea floors were flat and featureless. Heezen went to sea to collect soundings, bringing home stacks of sonar readings that needed analysis. Women were not allowed on board ships. Tharp’s job was to plot the depth and location of each ping thousands and thousands of times. She then turned these charts into graphs and diagrams that offered a visual representation of what the bottom of the sea looked like. And it was not flat and featureless.

At the time, the theory of continental drift, first proposed in 1912, was roundly ridiculed. Certainly, the continents could not move, and neither could the bottom of the ocean.

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Tharp’s analysis consistently showed a tall mountain range on the bottom of the Atlantic, bisected by a deep rift, suggesting continental drift was true. Heezen dismissed her findings.

Another man working in Tharp’s office was charting the location of the epicenter of earthquakes in order to help Bell Labs find the best spots to lay down transoceanic cables. His charts exactly mirrored Tharp’s charts. This correlation gave credence to the idea the crust of the Earth was pulling apart.

In 1956 when Heezen announced the discovery, and took credit for it, seismic waves rippled through the world of geology.

Tharp and Heezen mapped every ocean, finding the rift stretched around the globe, a distance of 40,000 miles (64,000 km).

National Geographic hired artist Heinrich Bernann to turn Tharp’s depictions into a panoramic painting of the topography of the world’s oceans. This image, released as posters inside the magazine in 1977, was pinned to the walls of classrooms worldwide and remains an instantly recognizable cartographic masterpiece.

Heezen was 53 when he died of a heart attack while working in a submarine mapping the sea floor near Iceland in 1977. Heezen and Tharp had been working together for nearly 30 years. His death effectively ended Tharp’s career. Though she remained a lowly assistant all her life, she received accolades and awards before she died in 2006. The Library of Congress named her one of the four greatest cartographers of the 20th century.

Today, “the Ring of Fire” is considered to be Earth’s largest physical feature. Satellite imaging effortlessly maps the ocean floor, while lasers measure the movement of the continents.