Magnetic field fluctuations at the earth's surface of ΔB/B~10-5 at <1 Hz are shown to result from motion with a peak velocity of 4 cm/s of ground water (~4 S/m) at ~5 km depth. Surface field changes can occur due to either divergence free fluid motion with tranverse spatial wavelengths of a few tens of km, and/or homogeneous flow which displaces local inhomogeneities in the earth magnetic field. ¿American Geophysical Union 1991