ILIAD builds on the assets resulting from two decades of investments in policies and infrastructures for the blue economy and aims at establishing an interoperable, data-intensive, and cost-effective Digital Twin of the Ocean (DTO). It capitalizes on the explosion of new data provided by many different earth sources, advanced computing infrastructures (cloud computing, HPC, Internet of Things, Big Data, social networking, and more) in an inclusive, virtual/augmented, and engaging fashion to address all Earth Data challenges. It will contribute towards a sustainable ocean economy as defined by the Centre for the Fourth Industrial Revolution and the Ocean, a hub for global, multistakeholder co-operation. The ILIAD DTO will fuse a large volume of diverse data, in a semantically rich and data agnostic approach to enable simultaneous communication with real world systems and models. Ontologies and a standard style-layered descriptor will facilitate semantic information and intuitive discovery of underlying information and knowledge to provide a seamless experience. The combination of geovisualisation, immersive visualization and virtual or augmented reality allows users to explore, synthesize, present, and analyze the underlying geospatial data in an interactive manner. The enabling technology of the ILIAD DTO will contribute to the implementation of the EU?s Green Deal and Digital Strategy and to the achievement of the UN Ocean Decade's outcomes and Sustainable Development Goals. To realize its potential, ILIAD DTO will follow the System of Systems approach, integrating all existing EU Earth Observing and Modelling Digital Infrastructures and Facilities To promote additional applications through ILIAD DTO, the partners will create the ILIAD Marketplace. Like an app store, providers will use the ILIAD Marketplace to distribute apps, plug-ins, interfaces, raw data, citizen science data, synthesized information, and value-adding services derived from the ILIAD DTO.