An instrument designed to check ground stability. It amplifies 2.5 million times, and can detect a rock movement as small as 0.000001 in (2.54 mu m). Receiving phones are placed in holes in the area being tested. Either earphones or automatic recording apparatus may be used for listening. A rate of 3 or more microseisms per second indicates probable collapse, and any rate over 25 or 30 per minute is considered dangerous. This instrument is also finding use above ground in checking highway cut slopes.