Ocean acidification driven by the uptake of anthropogenic CO 2 represents a major threat to ocean ecosystems, yet little is known about its progression beneath the surface. Here, we reconstruct the history of ocean interior acidification over the industrial era on the basis of observation-based estimates of the accumulation of anthropogenic carbon. Across the top 100 meters and from 1800 to 2014, the saturation state of aragonite (Ω arag ) and pH = −log[H + ] decreased by more than 0.6 and 0.1, respectively, with nearly 50% of the progression occurring over the past 20 years. While the magnitude of the Ω arag change decreases uniformly with depth, the magnitude of the [H + ] increase exhibits a distinct maximum in the upper thermocline. Since 1800, the saturation horizon (Ω arag = 1) shoaled by more than 200 meters, approaching the euphotic zone in several regions, especially in the Southern Ocean, and exposing many organisms to corrosive conditions.