Due to the low accessibility of the region, most of the seasonally ice-covered Southern Ocean remains unobserved during winter. Here we show that southern elephant seals (Mirounga leonina), equipped with oceanographic sensors, can measure winter hydrography with unprecedented temporal and spatial resolution. Seals provided a 30-fold increase in hydrographic profiles from the sea ice zone, allowing the major fronts to be mapped south of 60°S. Sea-ice formation rates in the East Antarctic sector were estimated from salinity increase in the upper water column and compared to simulations with a circumpolar finite-element sea-ice ocean model. From the observations, peak freezing rates of 2.5-3 cm/d were estimated for the period from late April to early May during the rapid northward expansion of the ice cover. While the pack ice becomes more compact, sea ice formation declines to 1 cm/d in June/July, and virtually stops by the end of August, when the maximum ice extent is reached. Modeled and observed freezing rates agree remarkably well, so that the model can be used to complete the seasonal cycle. By sampling the ocean during winter, elephant seals fill a blind spot in our sampling coverage, bringing us closer to a truly global ocean observing system.