The University of Tokyo, Japan, was approved to begin work on an interferometer detector in 2010. The detector will have arms 3km long (like Virgo) and will be built in tunnels in the Kamioka mine, 200m below the surface of Mount Ikenoyama. Excavation of the tunnels was completed in 2014. Building the telescope underground will reduce the influence of some noise sources, such as seismic and human activity, whilst cryogenically cooling the optical apparatus will also serve to reduce noise in the detector.
A collaboration between LIGO and three institutions in India, Raja Ramanna Center for Advanced Technology (Indore), the Institute for Plasma Research (Ahmedabad), and the Inter-University Centre for Astronomy and Astrophysics (Pune), have begun plans to build an interferometer detector in India. A detector in such a location would allow for much more accurate sky-localisation of gravitational wave sources over the entire sky. Even with joint runs of LIGO and Virgo, there would be large areas of sky where the detectors would not be able to narrow down the location very accurately without the help of a detector in the Eastern hemisphere.
For the much longer term, there is a European proposal, currently in the early design phase, for a third-generation detector called the Einstein Telescope. The detector would consist of a vacuum tube in the shape of an equilateral triangle, with sides of length 10km, inside which lasers would be projected at beam-splitters and mirrors in a similar manner to existing interferometer detectors. There would be six detectors; two interferometers along each arm, with three optimised for detecting high-frequency gravitational waves (10Hz – 10kHz) and the other three optimised for low frequencies (1Hz – 250Hz). The optical instruments will be cryogenically cooled, like KAGRA, to 10 Kelvin. The triangular design would allow for a separate measurement of the two gravitational wave polarisations in a signal.