Since 2004, several hundreds of diving marine animals, mainly Antarctic and Arctic seals, were fitted with a new generation of Argos-CTD tags developed by the Sea Mammal Research Unit of the University of St. Andrews in Scotland. These tags can be used to investigate simultaneously the at-sea ecology of these animals while collecting valuable oceanographic data. Some of these species are able to travel thousands of kilometres, continuously diving to great depths (590 ± 200 m, with maxima around 2000m). Through the years, these animals have become an essential source of temperature and salinity profiles (MEOP-CTD database available at http://www.meop.net), especially for the polar oceans, complementing efficiently the Argo array. One region where the use of instrumented seals has been particularly successful is the Antarctic continental shelf. Recent contributions to the study of the Antarctic Bottom Water production area near Prydz Bay, the rapidly-thinning ice shelves in Amundsen Bay, or the stratification in the marginal ice zone, are demonstrating the rapidly growing value of these data for Polar Research.