While many virtual educational environments are already in use, few attempt to mix the virtual and the real worlds to a very high extent. The benefits of such approach could be groundbreaking, leading to new paradigms of education, however the complexity of the challenges and the scale of the effort are still very high. The "3D UPB" platform takes this approach of a fully mixed reality environment to the scale of a large university campus with tens of thousands of users. This paper presents the concept and aims of “3D UPB”, its top level architectural components, the current stage of development and essential implementation details in order to point out the benefits and also challenges of this innovative educational and social platform. Preliminary evaluation, results and future work are also presented.