The VIRGO Challenge : Detecting Strain Amplitudes Smaller Than 10-21 G. Cagnoli, L. Gammaitoni, J. Kovalik, F. Marchesoni and M. Punturo J. Phys. IV France, 06 C8 (1996) C8-833-C8-836 DOI: 10.1051/jp4:19968179