The science ceases to feel abstract at a certain point, somewhere between the hum of the ship and the icy quiet of deep water. The meticulous process of mapping the Mariana Trench has been compared to “mowing the lawn” by James Gardner, a research professor at the University of New Hampshire’s Center for Coastal and Ocean Mapping. One meticulous strip at a time, overlapping sonar tracks are laid across thousands of square kilometers of seafloor. It sounds almost normal. It is not at all like that.
Near Guam, the Mariana Trench is located about seven miles below the surface of the Pacific Ocean. It is a hole in the Earth’s crust so deep that even if Mount Everest were dropped into it, more than a mile of water would still cover the summit. Using a multibeam echo sounder, an apparatus installed beneath the ship’s hull that sends sound pulses toward the seafloor and listens for what bounces back, Gardner and his colleague Andrew Armstrong led a mapping expedition that covered the roughly 400,000 square kilometers of the trench between August and October of 2010. Built pixel by pixel at a resolution of 100 meters per point, the resulting images showed a seafloor that had never been seen up close.
Even Gardner’s team was taken aback by what they discovered. One ridge may have crossed the trench floor, according to satellite imagery. Four of them, tectonic bridges that rose up to 2,500 meters above the base of the trench as a result of the slow, grinding collision of the Pacific and Philippine tectonic plates, were verified by the sonar data. The younger Philippine plate is being pushed beneath the roughly 180 million-year-old Pacific plate, which is becoming denser with age. Instead of subducting cleanly as it descends, seamounts on the Pacific plate become trapped against the trench wall, creating these striking cross-ridges. Gardner remarked, “That got me excited,” following his presentation of the results at the San Francisco meeting of the American Geophysical Union. A researcher’s statement that something “got me excited” in that detached, measured tone seems to indicate that something noteworthy occurred.
The expedition also yielded the most accurate measurement of Challenger Deep, the lowest point in the trench, at the time: 10,994 meters, with a 40-meter margin of error. That qualification is important. Even tiny mistakes compound when calculating depth at eleven kilometers using acoustic signals that can be slowed and bent by the fluctuating pressure and temperature of the water column above. “When you’re dealing with something that’s 11 kilometers deep, you have to deal with inherent uncertainties in the system,” Gardner said. Future measurements might improve the figure even more, and there is still legitimate scientific disagreement regarding the precise location and depth of the deepest point.
An entirely different type of explorer was drawn to that debate. As part of his “Five Deeps” expedition, which aims to reach the lowest points of all five ocean basins, Victor Vescovo, a private ocean explorer and owner of Caladan Oceanic, made eight trips to Challenger Deep starting in 2018. Vescovo’s team collaborated with NOAA to share sonar data and depth profiles from the dives, providing NOAA Corps Commander Sam Greenaway with a dataset that, in his words, provided at least a five-fold improvement in measurement precision over previous methods. Kathryn Sullivan, a former NOAA administrator and NASA astronaut, and Kelly Walsh, the son of Don Walsh, who first went down to the trench floor with Jacques Piccard in 1960, were among Vescovo’s guests on these dives. The fact that the son of the first person to see Challenger Deep is returning to view it in a submersible that wasn’t around when his father made that initial dive, sixty years later, has a subtly poetic quality.
When asked why any of this matters, Greenaway said succinctly, “I think it’s basic human curiosity.” A seven-year-old might ask, “How deep is the ocean?” Although it is easy to write off that framing as being overly simplistic, it actually addresses a real issue. Only 43% of America’s 3.4 million nautical square miles of underwater territory have been fully mapped to current standards, according to the national strategy created by the U.S. National Ocean Policy Council.
The publicly accessible data being collected at the Mariana Trench directly contributes to our understanding of climate systems, continental shelf boundaries, earthquake subduction zones, and a host of other issues that go far beyond simple curiosity in terms of economic and security implications. Although it’s still unclear how soon that mapping objective will be accomplished, collaborations between organizations like UNH, NOAA, and private explorers like Caladan indicate that the pace is at least quickening.
You don’t remember the depth records or the technology as you watch this work take shape. It’s the picture of scientists on a ship using sonar pings to run overlapping lanes across an ocean abyss, creating a picture of a place that has never seen light, and being genuinely shocked by what they discover.
