During World War II, oceanographer Harry Hess and his crew, aboard the USS Cape Johnson, scoured the western Pacific Ocean in search of enemy submarines.

By timing echoes generated by sonar “pings,” or sound waves, they measured the depth of the ocean in hopes of finding minor discrepancies in data that might indicate a submarine’s presence. But those readings also revealed something much bigger—seamounts, or underwater volcanic mountains rising more than 1,000 meters (3,300 feet) from the seafloor.

The vast information Hess and his team collected laid the foundation for continued seafloor exploration. With Hess's data as a starting point, American geologists Marie Tharp and Bruce Heezen published some of the first detailed maps of the ocean floor in the 1950s and '60s. They provided visual evidence for what became Hess’ theory of seafloor spreading, in which tectonic plates in the Earth move apart at mid-ocean ridges and form new crust. It helps explain geologic phenomena, ranging from continental drift to earthquakes to the formation of mountains.

“It was a paradigm shift in our understanding of Earth processes,” says Larry Mayer, professor and director of the Center for Coastal and Ocean Mapping at the University of New Hampshire. “It was the first time one theory really explained almost all of the processes we see in the ocean and on land.”

As British explorer James Clark Ross embarked on a voyage to chart the Antarctic coastline in 1840, he made some of the earliest attempts to measure the depth of the ocean. But a lack of advanced technology often resulted in inaccurate measurements. Initially, scientists tried to measure seafloor depth by lowering a weighted hemp rope over the side of their ships and measuring its length. In 1872, a newly invented piano-wire sounding system allowed for quicker and more reliable calculations. 

Systematic efforts to map the seafloor continued during the British-led, 1,000-day voyage of the HMS Challenger. In addition to providing new data about marine life and physical and chemical deep-sea conditions, the expedition’s findings hinted at the presence of a mountain chain dividing the eastern and western halves of the Atlantic Ocean.

“Certainly, before they developed acoustic soundings—which is sonar—there’s a lot of uncertainty in the depth of the seafloor because the methods weren't good enough,” says Heidi Dierssen, marine sciences professor at the University of Connecticut. “Once they discovered that they could use the speed of sound pretty accurately, it really revolutionized our understanding of Earth itself.”

According to the National Oceanic and Atmospheric Administration, the use of acoustic sounding systems began during World War I and the Coast and Geodetic Survey was regularly using sonar to map deep-water areas by the 1920s.

During World War II, the United States paused most of its seafloor mapping efforts. The exception came when Hess, then a young Princeton University professor, joined the U.S. Navy as captain of the USS Cape Johnson. Equipped with the latest sonar technology, Hess was able to collect an unprecedented amount of data, rapidly and around the clock, as he and his crew searched for enemy submarines.

“It allowed [for] continuous ‘pings’ in the water as the ship moved around different places,” says John Orcutt, geophysics professor at Scripps Institution of Oceanography at the University of California, San Diego and former U.S. Navy submariner. “So you started getting a two-dimensional view of what the seafloor looked like.”

Until the war, scientists largely believed the ocean floor was “a flat, unchanging plain, a dumping ground slowly filled by sediments eroding from land,” Tharp wrote in a 1999 book. But Hess encountered large geological features, including deep-sea trenches and 160 seamounts between the Hawaiian and Mariana islands. Hess called them guyots, named for Swiss geographer Arnold Guyot, who founded Princeton’s geology department in the mid-1800s. 

This finding led to new questions about the mobility of the seafloor, and the United States launched more deep-sea expeditions starting in the late 1940s. 

Tharp, who began working at the Lamont Geological Observatory (now the Lamont-Doherty Earth Observatory at Columbia University) in 1948, famously drew comprehensive seafloor maps as her colleague, geologist Bruce Heezen, collected the data aboard research ships. (Because of her gender, Tharp initially wasn’t allowed on the ships that collected the seafloor data that she used to make her maps—and she didn’t set foot on a research cruise until 1968.) Tharp and Heezen published a comprehensive map of the Atlantic Ocean in 1957 and the first detailed world map of the ocean floor by 1977. 

“We continued on, from one sounding to the next, and one ocean to the next,” Tharp wrote. “We weren’t daunted by the tens of thousands of soundings we had to plot.” 

Their maps showed the existence of volcanic mountain chains extending over 40,000 miles worldwide, laying the foundation for the theory of plate tectonics. As Dierssen explains, “The ocean has these areas that form brand-new crust—we call them mid-ocean ridges in the Atlantic. When you create brand-new crust, you have to get rid of old crust, or else the Earth would expand.” The older, denser plates descend into the Earth's mantle at subduction zones, characterized by deep ocean trenches.

Tharp’s maps were widely circulated in scientific communities and published in National Geographic, which commissioned Austrian artist Heinrich C. Berann to reproduce map-paintings of the ocean floor. The Hartford Courant described the maps in 1968 as a “geological-oceanographic landmark which will be of inestimable value to future submariners while it unveils another of Earth’s mysteries.”

In 1912, German scientist Alfred Wegener had proposed the theory of continental drift, in which continents slowly moved apart over time. But the scientific community was largely skeptical. That started to change when, in 1960, Hess shared his theory of seafloor spreading—which occurs at mid-ocean ridges—as the mechanism driving this movement, plus many other geological processes. The work of Tharp and Heezen provided vital evidence.

“Without that mapping, we never would have known about plate tectonics, and the other evidence really fills it in,” Mayer says. “But it was the shape of the seafloor, and what was up and what was down, that told the picture. And that continues on today.”

Modern mapping techniques—including high-resolution multibeam sonar—offer scientists a three-dimensional picture of the deep sea, Mayer says. As of June 2024, about 26 percent of the ocean floor is currently mapped. But as technology advances, scientists continue making discoveries that are vital for predicting natural disasters and understanding a multitude of phenomena across the planet.