An array of 25 three-component magnetometers was operated in South-West Africa, Botswana and north-western Rhodesia from December 1971 to February 1972. Results from three periods of geomagnetic disturbance are presented. Magnetograms and maps of Fourier transform amplitudes and phases show a large anomaly in the vertical and northward horizontal variation fields over the period range 21-171 min. A well-defined reversal in the vertical field is observed between Outjo (OUT) and Sukses (SUK) on the western edge of the array. Transfer functions between mean horizontal field components and station vertical components show that a conductor that trends east-west in the western part of the array and curves north-eastward in the east underlies the array. A study of the half-widths of normalized anomalous profiles and the induction vectors indicates that the conductor is a crustal structure. It is most probably connected to the African rift system. A separation of the in-phase and quadrature-phase induction vectors indicates a structure with a high inductive reactance off the western edge of the array.