DEEP proposes to develop a novel, Exascale-enabling supercomputing platform along with the optimisation of a set of grand-challenge codes simulating applications highly relevant for Europe's science, industry and society. The DEEP System will realise a Cluster Booster Architecture that can cope with the limitations purported by Amdahl's Law. It will serve as proof-of-concept for a next-generation 100 Petaflop/s PRACE production system, striving for independent provision of HPC technology, in particular general purpose Exascale performance supercomputers in Europe. The DEEP concept is based on the duality of an advanced multi-core Cluster system with InfiniBand interconnect complemented by a Booster of Intel many-core MIC processors connected through a Terabit EXTOLL network. A novel open source system software stack along with Cluster Booster adapted programming models, libraries, and performance tools will achieve high productivity and will enable unprecedented scalability on millions of cores. The DEEP hardware and software technology is developed in Europe while the new many-core processor is an essential component of international cooperation. The DEEP concept has the potential to improve the power efficiency of HPC systems by an order of magnitude. Its innovative cooling concept will allow approaching power usage effectiveness values very close to 1. Representative HPC codes from Health and Biology, Climatology, Seismic Imaging, Industrial Design, Space Weather, and Superconductivity will be optimised on DEEP and the extrapolation to millions of cores will be demonstrated. The pan-European DEEP consortium has proven competence to meet the project's massive technological and scientific challenges. DEEP aims at disseminating knowledge amongst major European industrial stakeholders and the entire PRACE consortium through its technical advisory group STRATOS and will contribute to the vision of the PROSPECT association for a European HPC technology platform.