This project concerns the study and the conceptual design for a new infrastructure that will bring Europe to the forefront of the most promising new development in our quest to understand the history and future of the Universe, the emergence of the field of Gravitational Wave Astronomy. We are proposing to study the conceptual design of a new European infrastructure, a GW observatory that could cover the complete frequency range from about 1 Hz to 10 kHz that is potentially observable from ground. We are aiming at the identification of the possible technologies and design characteristics that could lead to an amplitude sensitivity more than a hundred times that of the existing first generation devices. This will increase the observable volume of the universe and, correspondingly, the event rate by a factor of more than one million. This formidable task will be achieved by combining all presently known scientific breakthroughs of measurement science in a single observatory. In fact, to realize such a third generation gravitational wave detector a formidable, but possible, progress in non-classical light, advanced lasers emitting hundreds of Watt of continuous power, novel signal enhancing techniques, interferometry by using diffractive optics, cryogenic cooling of critical optical components, vibration isolation techniques, monolithic suspensions for the optical components is required.