In 2012, James Cameron went in a submarine down to the floor of the Mariana Trench, one of the deepest parts of the world's oceans. He says he dove down 10,908 meters. Last May, Victor Vescovo went down into the Trench -- and reached 10,924, precisely 16 meters deeper.
But as Matt Simon writes in Wired, precision is incredibly hard to measure in waters that deep -- so there's still quite an argument about who went deeper.
Why is it hard to measure things down that far? Well ...
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... if you wanted, you could drop a 11,000-meter-long cable down into the Challenger Deep and measure depth that way, but the thing will be buffeted by 7 miles of currents, obliterating any pursuit of accuracy.
Instead, scientists and explorers typically rely on sound or pressure to measure depth, or both. Pressure, of course, increases as you go deeper. “Pressure is probably the best way to get an absolute measure of depth,” says Mark Zumberge, a research geophysicist at the Scripps Institution of Oceanography. But that alone won’t suffice, because water pressure can fluctuate as you descend—it depends in part on the water’s density, which changes up and down the water column based on temperature and salinity.
“To convert pressure to depth, you need to know the water density over the full water column and also the local value of gravity, which varies by about half a percent over the surface of the Earth,” Zumberge says. And if you’re trying to be really precise, it’s worth noting that gravity “even varies by a couple hundredths of a percent from the sea surface to the bottom of the ocean.”
The other way to measure depth is using sonar, but that comes with its own complications.