Geomagnetic micropulsations in the Pc 4 frequency band which are highly monochromatic and have amplitude-modulated wave forms have been termed 'giant pulsations' and have been observed to reach amplitudes of several tens of nanoteslas. In this paper, four such events are analyzed by using meridian line and two-dimensional magnetometer networks. It is found that these pulsations are highly localized in latitude and have a large azimuthal wave number k⊥, although the large magnitude of k⊥ does not necessarily reflect the longitudinal extent of the disturbed region. While such events are extremely rare, they may occur on 2 or more consecutive days. The pulsations occur on quiet days and normally after a sustained period of low magnetospheric activity. The initiation of substorm activity appears to quench the giant pulsations, which may return after the substorm has subsided. We suggest that the pulsations accompany suddenly reduced levels of magnetospheric convection, and indeed the source region of the pulsations moves poleward during the course of the pulsational activity, suggesting a recovery phase relationship. It is suggested that the Pc 4 giant pulsations may be due to the occurrence of field line resonances at the plasmapause in the region where the electric field changes its azimuthal direction from westward to eastward.