Earth breathes in and out, murmuring gently to itself as it does so. The habit has been ascribed to the tickling effects of ocean waves - but a new-found twisting oscillation might reopen the search for the source. Over the past decade, the word 'hum' has acquired a special meaning for seismologists. No longer just what they might do under the shower, it connotes for them a fundamental resonant oscillation of the Earth. A sequence of these oscillation modes, with periods of between around 2 and 5 minutes, was first identified in 1998. These were all 'spheroidal' modes, representing perturbations of the planet's equilibrium surface, rather akin to the effect of waves on water. Writing in Geophysical Research Letters, Kurrle and Widmer-Schnid-rig now introduce a further, entirely different mode - 'toroidal' hum, in which parts of Earth's surface twist around in the horizontal plane (Fig. 1, overleaf).
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