A mushroom-shaped hydrogen cloud rearing 1000 light years above the plane of our galaxy is the aftermath of a massive gas cloud that dive-bombed the Milky Way, new computer simulations suggest. The work explains why the cloud is unlike any other found so far. The cloud, called GW 123.4-1.5, was discovered in 1999 by Jayanne English of the University of Manitoba in Winnipeg, Canada, and colleagues, who found it in a radio survey of the galactic plane. Two explanations for its familiar shape were offered at the time – that the mushroom is a bubble of gas blown out by...