Over the last decades, sea-ice in the Arctic Ocean (AO) has undergone unprecedented changes, with drastic decline in its extent, thickness and duration. Modern climate models are unable to simulate these changes, leading to large uncertainties in Arctic and Global Change predictions. Sea-ice strongly attenuates solar radiation and it is generally thought that phytoplankton, which drives Arctic marine CO2 sequestration, only grow in open waters once sea-ice retreats in spring. However, the discovery of large under-ice phytoplankton blooms (UIBs) growing beneath sea-ice contradicts this paradigm. UIB productivity in ice-covered regions has been suggested to be ten-fold larger than presently modelled. By initiating an international network (USA, France, Canada, Germany, Norway), the CAP-ICE project will acquire knowledge on the occurrence of UIBs, the physical mechanisms that control their initiation and productivity, and will quantify how UIBs affect the Arctic carbon cycle and climate. CAP-ICE will equally combine observational, modelling and novel technology approaches. Multiple pan-Arctic expeditions will provide new field observations on the environmental conditions controlling UIBs. Since UIBs are invisible to ocean colour satellite sensors, developing a novel model adapted to under-ice environments will allow quantifying the contribution of UIBs to the Arctic carbon cycle. Finally, the recent launch of autonomous robotic platforms (Bio-Argo floats) will support the first assessment of UIB primary production and carbon export in AO and the implementation of a Bio-Argo Arctic network.