LIGO scientist here. The way this is presented can be a little deceptive - the isolation is very frequency dependent. At high frequencies (&gt;10Hz), the pendulums and blade springs in the suspension isolate the mirrors very well, so they are moving by only these small amplitudes (10^-19 meters). But at low frequencies (&lt;1Hz) the isolation ratio is essentially 1, so the amplitude of the mirror motion is roughly the same as that of the ground (about 10^-6 meters).