Below a few tens of hertz interferometric detection of gravitational waves is masked by seismic vibrations of the optical components. In order to isolate the mirrors of the VIRGO interferometer, a sophisticated suspension system, called superattenuator, has been developed. Its working principle is based on a multistage pendulum acting on seismic vibrations as a chain of second order mechanical low-pass filters. A complete superattenuator has been built and tested. This apparatus allows extending the VIRGO detection band down to a few Hz. A detailed description of the attenuation system and its performance are presented in this article.

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