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. In this
review, the properties of the most investigated single atom magnet, namely a Ho atom adsorbed on an ultrathin
MgO film, will be presented 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.