Ocean acidification driven by the uptake of anthropogenic CO₂ 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 (OIA) from 1800 to 2014 on the basis of observation-based estimates of the accumulation of anthropogenic carbon. Across the top 100 m and over the industrial era, the saturation state of aragonite (Ω_arag) and pH = -log[H⁺] decreased by more than 0.6 and 0.1, respectively, with a progress of nearly 50% over the last 20 years (1994-2014). While the magnitude of the Ω_arag change decreases uniformly with depth, the magnitude of the pH decrease exhibits a distinct maximum in the upper thermocline. Since 1800, the saturation horizon (Ω_arag=1) shoaled by more than 200 m, approaching the euphotic zone in several regions, especially in the Southern Ocean, and exposing many organisms to corrosive conditions.