Swells radiating across ocean basins are fingerprints of the large ocean storms that generated them, which are otherwise poorly observed. Here we analyze the signature of one swell event in the seismic noise recorded all around the Pacific and we show that it is a natural complement to the global coverage provided by the Synthetic Aperture Radar wave mode data from ENVISAT. In particular the seismic stations are much more sensitive to low frequency and amplitude signals than buoys and SAR, capturing swell forerunners a couple of days before they can be detected from space or in situ data. This information helps detect in the SAR measurements the presence of very long swell, with periods of 22 s in our case example, that were otherwise excluded. Citation: Husson, R., F. Ardhuin, F. Collard, B. Chapron, and A. Balanche (2012), Revealing forerunners on Envisat's wave mode ASAR using the Global Seismic Network, Geophys. Res. Lett., 39, L15609, doi: 10.1029/2012GL052334.