Oceanographers are fond of saying that we know more about the moon's surface than we do about Earth's seafloor. It's true. As of 2017, only 6 percent of the global seabed had been mapped, typically by ships with sonar instruments sailing back and forth in straight lines across a local section of sea.

But since then, nations have become eager to chart the seafloor within their own “exclusive economic zones,” which reach 200 nautical miles from their shores, in part to look for critical minerals they can scrape up using big mining machines. The other push is Seabed 2030—an effort to map Earth's entire seafloor by 2030, run jointly by the Nippon Foundation and the nonprofit General Bathymetric Chart of the Oceans.

The goal is to collect and stitch together mapping done by governments, industries and research institutions everywhere. Public release of previously private bathymetric data is helping to widen the areas plotted. And uncrewed, remotely operated vehicles fitted with sonar that can zoom around underwater for days at a time are speeding the pace of mapping. By June 2022 an impressive 21 percent of the world's seafloor had been charted. The more experts map, the more surprises they find—such as the three unexpected, unusual formations revealed here.