Although the mechanical influence o

Although the mechanical influence of seismic vibrations can be mitigated by suspending the optics through suitable number of attenuation stages, the direct coupling between the moving ground and the optics through Newtonian gravitational attraction cannot be shielded. This noise, called the Newtonian noise or gravity gradient noise, is the ultimate limitation on gravitational wave detection on the Earth. It can be reduced by moving the interferometer underground, where the optics are less under the effects of the gravity gradient field of surrounding rocks. The Newtonian noise can also be calculated and subtracted if motion of the ground surrounding the optics is measured, typically using a large number of sensors. More detailed studies on Newtonian noise are found in reference [17].