A Planetary Stargate is the fourth panel in the mural.
This panel shows one of the main elements of an advanced civilization, a planet size stargate. Don't just take your home with you to a new solar system, take your Home-World!
The other big element here is the volcano, that may or may not have been active in Shroomy's Time. Can't really tell with psychodelic mushroom eating rhinos.
Click the image for pop up.