In order to detect the rare astrophysical events that generate gravitationalwave (GW) radiation, sufficient stability is required for GW antennas to allowlong-term observation. In practice, seismic excitation is one of the mostcommon disturbances effecting stable operation of suspended-mirror laserinterferometers. A straightforward means to allow more stable operation istherefore to locate the antenna, the ``observatory'', at a ``quiet'' site. Alaser interferometer gravitational wave antenna with a baseline length of 20m(LISM) was developed at a site 1000m underground, near Kamioka, Japan. Thisproject was a unique demonstration of a prototype laser interferometer forgravitational wave observation located underground. The extremely stableenvironment is the prime motivation for going underground. In this paper, thedemonstrated ultra-stable operation of the interferometer and a well-maintainedantenna sensitivity are reported.
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