What’s happening with the fringe of sea ice surrounding Antarctica? Is it growing or shrinking, and why are there strikingly different trends in different regions of the continent? To understand a puzzle that has been confounding scientists in recent years, researchers need to know not only the spatial extent of the ice, but also how thick it is, which is tricky to estimate from satellite data alone or from scant point measurements taken by drilling into ice floes. Now, a team of scientists is offering a third way of collecting that data: from under the ice itself. The researchers used an autonomous underwater vehicle (AUV) named SeaBED that is armed with upward-looking sonar to map the underside of the sea ice floes. During two different research expeditions, in the mid- to late springs of 2010 and 2012, the AUV—which resembles a 2-meter-long bunk bed with twin hulls stacked on top of one another—traveled back and forth through several different Southern Ocean waters in a lawn mower–like pattern at depths of 20 to 30 meters under the ice to collect a 3D survey of the topography of the sea ice’s underbelly. After compiling 10 floe-scale maps of the ice from the Weddell, Bellingshausen, and the Wilkes Land regions of the continent, the researchers found that the sea ice thickness tended to be highly variable, with many ridges and valleys, they report online today in Nature Geoscience. On average, Antarctic sea ice may be considerably thicker than once thought, which could significantly change how scientists assess sea ice dynamics and their interactions with the ocean in a warming world.