Correction to: Nature https://doi.org/10.1038/4441023a, published 21 December 2006.

In this Brief Communication, the scale bar in Fig. 2a was inadvertently changed from ‘10 cm’ to ‘0.01 m’. The Supplementary Information and text have the correct value. We thank Robert Menzies-Wilson and Jeroen Bergmann for bringing this error to our attention. The original Brief Communication has not been corrected online.