On 30 October 2003, an ongoing geomagnetic superstorm knocked down a part of the high-voltage power transmission system in southern Sweden. The blackout lasted for an hour and left about 50,000 customers without electricity. The incident was probably the most severe geomagnetically induced current (GIC) failure observed since the well-known March 1989 Qu¿bec blackout. The three-phase storm produced exceptionally large geomagnetic activity at the Fennoscandian auroral region. Although the diversity of the GIC drivers is addressed in the study, the problems in operating the Swedish system during the storm are attributed geophysically to substorms, storm sudden commencement, and enhanced ionospheric convection, all of which created large and complex geoelectric fields capable of driving large GIC. On the basis of the basic twofold nature of the failure-related geoelectric field characteristics, a semideterministic approach for forecasting GIC-related geomagnetic activity in which average overall activity is supplemented with statistical estimations of the amplitudes of GIC fluctuations is suggested. The study revealed that the primary mode of GIC-related failures in the Swedish high-voltage power transmission system were via harmonic distortions produced by GIC combined with too sensitive operation of the protective relays. The outage in Malm¿ on 30 October 2003 was caused by a combination of an abnormal switching state of the system and tripping of a low-set residual overcurrent relay that had a high sensitivity for the third harmonic of the fundamental frequency.