The High Arctic is assumed to be the last reserve of a stable cryosphere. Recent publications from the Canadian High Arctic indicate, however, that changes are currently happening much faster than expected, and the future trajectory is unknown. The rate and magnitude of recent climate warming has implications for the stability of ice, especially in the far north. Moreover, air temperatures at unprecedently high levels lead to drastic retreat in sea ice extent, larger and longer ice-free shelf seas, which now may act as a “new Arctic” moisture source with clear impact on the regional hydrological and cryospheric processes. Terrestrial ice (glaciers and ice caps, ice fraction in permafrost) in the high Russian Arctic is expected to change. This terrestrial ice is a freshwater inventory on land or that has been preserved in a stable state for hundreds of years. It contains unique information about past and present climate changes which is stored in the terrestrial ice to be studied, i.e. with stable isotope methods. In this presentation, we focus on the stable hydrogen and oxygen isotopes as a unifying proxy employed for addressing cryospheric, atmospheric, marine and terrestrial research questions In the framework of the Arctic Century 2021 expedition, we addressed the hydroclimate situation of High Arctic Islands, by studying firn/ice cores from three visited ice caps. Here, Windy Ice Dome (WD, Franz-Josef-Land) and Akademii Nauk ice cap (AN, Severnaya Zemlya) have been revisited and cored after more than 20 years, now complementing earlier longer ice cores. The University ice cap (UN, Severnaya Zemlya) has been cored for the first time. Key observations were intensive near-surface melt processes inducing the percolation of meltwater, numerous thick melt layers (reaching up to 80 cm), as well as liquid water in the borehole at AN and UN ice caps complicated drilling. The ice-coring program (5 cores, 27.2 m of core) has been accompanied by snow sampling (N=126) and an extensive hydrological program including 170 sea water samples, 16 terrestrial surface waters and 31 precipitation samples. Moreover, ground ice samples were retrieved from a permafrost exposure at Severnaya Zemlya. The Arctic Century Expedition took place in August/ September 2021 and was jointly organized by the Swiss Polar Institute, the Arctic and Antarctic Research Institute, Russia and the Helmholtz Centre for Ocean Research GEOMAR in Germany.