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Starting in 1972, persistent large-amplitude magnetic signals were observed in recordings of ultra-low-frequency (ULF, frequencies less than 5 Hz) geomagnetic field fluctuations made on the Stanford campus. The energy of these ULF signals is concentrated predominantly at frequencies below about 0.3 Hz and their amplitude, which has increased greatly since 1972, is at least ten times greater than the normal natural background level of geomagnetic activity, i.e., it is comparable to the level observed during great geomagnetic storms. ULF magnetic field measurements at additional locations away from the Stanford campus have enabled us to identify the San Francisco Bay Area Rapid Transit (BART) system as the source of these large-amplitude magnetic fields, which appear to be produced throughout the entire Bay area and vicinity (area ≈100 km2). Primary power for the BART trains is provided by a 1000 V dc third rail (total length ≈270 km) and a single loaded 10-car train can draw as much as 7 Mw during acceleration. Large fluctuating currents (5 to 10 kA) therefore flow in the third rail and associated conductors, with the result that the entire BART system constitutes a large ULF “antenna.” Similar systems planned or already in operation in other parts of the US (e.g., the Washington, DC, Metro) will probably also produce similar large-amplitude ULF electromagnetic fields.