  We describe a fiber optical gyroscope based on the Sagnac effect realized on
a multiplexed telecom fiber network. Our loop encloses an area of 20 km^2 and
coexists with Internet data traffic. This Sagnac interferometer achieves a
sensitivity of about 1e-8 (rad/s)/sqrt(Hz), thus approaching ring laser
gyroscopes without using narrow-linewidth laser nor sophisticated optics. The
proposed gyroscope is sensitive enough for seismic applications, opening new
possibilities for this kind of optical fiber sensors
