Complete Surprise

I tested my (self-designed) magnetometer on the beach this morning with son M -- see the photo -- and there was a complete surprise. The magnetometer suffered total overload -- from changing magnetic fields. But that should be impossible! I first suspected the sand (pictured) -- but the sand was perfectly still. Perhaps this was induced in the sand? But how? We moved off the beach. Still my magnetometer suffered total overload. Perhaps it was the fault of the magnetometer? We moved onto the mountain. Now my magnetometer was back to normal. OBSERVATION: I searched the Internet. In 2003, New Scientist first reported that induced magnetic fields had been found in the ocean, 6 000 times smaller than the earth's magnetic field. This is well within range of my magnetometer. Then, a few months ago, the European Space Administration revealed that these fields measured 2.0 to 2.5 nT at satellite altitude. My magnetometer is that sensitive at ground level. The only thing moving near my magnetometer was the waves gently lapping on the shore -- and small ocean currents, maybe. Is that what I detected? What adds to the interest is that not many magnetometers are designed like mine -- to detect changing magnetic fields.