KM3NeT is a deep-sea research infrastructure to be constructed in the Mediterranean Sea hosting a neutrino telescope with a volume of at least one cubic kilometre. The scientific case for a neutrino telescope of a cubic kilometre scale is overwhelming. The infrastructure it requires will be shared by a host of other sciences, making continuous and long-term measurements in the fields of oceanography, geophysics, and marine biological sciences possible. The feasibility of neutrino astronomy with a detector in the deep sea was proven by the successful deployment and operation of the ANTARES prototype detector. The potential of the detection technique, based on the reconstruction of the tracks of muons, the possible reaction products of the sought after neutrinos, has been demonstrated. With two other pilot projects, NEMO and NESTOR, different detector configurations and techniques were explored. The three projects have provided a wealth of information on the technologies required for a large deep-sea neutrino telescope. KM3NeT will reap the benefits. It is planned to make KM3NeT a CO 2 -neutral facility, using wind or solar energy to supply the required power for the underwater system as well as the shore station. The proposed infrastructure will be built by a European consortium (KM3NeT). The total cost is estimated at 220-250 Mh.