MAR-CO2 aims to develop fit-for-purpose MRV systems for mCDR, enabling safe, verifiable, and sustainable climate mitigation. The project’s core objective is to co-design, refine, and test MRV tools to quantify carbon removal additionality and durability, and assess ecological risks and co-benefits, along with the governance frameworks needed to ensure transparency, fairness, and public trust. These tools are tested in real-world conditions, moving beyond lab-scale studies to assess feasibility in the context of operational mCDR through industry partnerships. MAR-CO2 integrates interdisciplinary expertise from the fields of oceanography, modelling, ecology, social science, and governance, enabling the investigation of mCDR as a complex and interconnected human-environment system rather than simply a technological one. Advanced observational technology, experimental approaches, and multi-dimensional modelling help build a comprehensive understanding of mCDR and its impacts, and are used to develop a novel Digital Twin platform for simulating mCDR “what if” scenarios. Social perceptions are regularly queried and governance systems stress-tested to ensure MRV tools are socially just and governable. A co-design approach is deeply embedded in the project, with stakeholder mapping, public deliberation, and governance scenario stress-testing ensuring that the MRV systems developed in MAR-CO2 balance scientific rigor, accuracy, and equity with resource efficiency, governability, and transparency. MAR-CO2 contributes to EU climate leadership by supporting the integration of mCDR into climate strategies (e.g., CRCF, EU ETS), aligning with biodiversity protection goals, and advancing the European Digital Twin of the Ocean. By interfacing a team of prominent scientists with diverse public-sector and governance actors, and by using industry-led