Einstein Telescope is a concept for a larger LIGO-like gravitational wave observatory, with arms 10km in length. Like KAGRA, it is to be underground.
The design has three arms in an equilateral triangle, for three Michelson interferometers using 60 degree angles. And there will be two such sets of interferometers using the same tunnels (totaling six interferometers in all), each set-of-three tuned so as to optimize for a different frequency.
The length contributes to the sensitivity, the goal being to detect more sources such as those more distant, those weaker at the source, and at frequencies for which LIGO is marginal.