Galactic Worm 123.4-1.5 (GW 123.4-1.5) is an

H I region An HI region or H I region (read ''H one'') is a cloud in the interstellar medium composed of neutral atomic hydrogen (HI), in addition to the local abundance of helium and other elements. (H is the chemical symbol for hydrogen, and "I" is the Ro ...

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 High-velocity clouds (HVCs) are large accumulations of gas with an unusually rapid motion relative to their surroundings. They can be found throughout the galactic halo of the Milky Way. Their bulk motions in the local standard of rest have veloc ...

in question is theorized as having hit at around 100 km/s, 5 years ago.

References

H I regions Cassiopeia (constellation) {{nebula-stub