Spheroidal fundamental mode oscillations of the Earth for frequencies between 2 and 7 mHz (millihertz) are observed even on seismically quiet days. Two hypotheses of the cause of these oscillations are investigated: the cumulative effect of small earthquakes and atmospheric pressure variations. The cumulative effect of earthquakes, assuming that earthquakes follow the Gutenberg-Richter law, is shown to be 1--2 orders of magnitude too small. The observed amplitudes of modes require an equivalent earthquake of magnitude 6.0 everyday, which cannot be achieved by summing up contributions from small earthquakes. The hypothesis of atmospheric excitation is favored because of the discovery of seasonal variations in stacked modal amplitudes for spheroidal modes between 0S20 and 0S40. It is also evaluated by comparing observed modal amplitudes with theoretical amplitudes, derived from a stochastic normal mode theory. The source of excitation is atmospheric pressure variations, which indicate turbulent motion of the atmosphere for the frequency range of interest and are estimated by barometer data. The observed modal amplitudes can be matched by the stochastic normal mode theory, indicating that atmospheric pressure variation is large enough to excite solid Earth normal modes up to the observed amplitudes. Therefore two lines of evidence, detection of seasonal variations and approximate match of overall modal amplitudes, support the hypothesis that the continuous background oscillations are excited by atmospheric pressure variations. ¿ 1999 American Geophysical Union