Surface-supported single atoms with long magnetic lifetime, also called single atom magnets, represent the ultimate limit of downscaling of a magnetic bit. Due to the possibility to write, store and read information in their magnetic states, these atoms are model systems for future ultra-high-density magnetic storage devices. This review presents the properties of the most investigated single atom magnet, namely a Ho atom adsorbed on an ultrathin MgO film, together with the techniques used to address its quantum level structure and magnetic stability. The temperature and field dependent relaxation mechanisms of this unconventional magnetic system will be discussed based on recent findings. Finally, an overview of the open issues and possible future directions of the research will be provided.