FIGURE 1. Supernova supershell exposed.

The most spectacular known supershell, produced by 100 or more supernovae, is revealed by this map¹² of emissions of the atomic hydrogen line at a wavelength of 21 cm, the tracer used by Koo and colleagues². The width of the image covers 20° of the sky; brighter yellow corresponds to higher and darker green to lower hydrogen density. The combined explosive energy has lifted the entire Galactic plane (white portion at the bottom) and sent shards of gas flying about around 6,500 light years into the Galactic halo, from which they will eventually return. (Figure courtesy of Y. Pidopryhora.)