GW 123.4 1.5

Galactic Worm 123.4-1.5 (GW 123.4-1.5) is an H I region with a mass of approximately 10⁵ M_Sun. It has an unusual mushroom-shaped structure that may be the result of having been formed by an infalling cloud slamming through the disc of the Milky Way from the other side. The high velocity cloud in question is theorized as having hit at around 100 km/s, 5×10⁷ years ago.