The horizontal striation on the walls is caused by the flow of lava. At one point the tube was a channel for magma flowing from some volcanic source, like water through a straw. As the flow slows the lava cools and flattens, which forms that relatively smooth floor. Once you clean it up it makes a pretty nice dance hall I guess.
Utterly pointless backstory: Keep Mars Red is based off this campaign/ popular California bumper sticker :B It’s a fake Earth organization I made up that petitions research/ habitation on Mars, in order to keep it pristine from human waste and meddling. Nobody takes their cause seriously. Mike’s old research partner, Braid, put the poster up as a passive-aggressive joke because Mike is a slob and never picks up after himself.
Today’s bonus art: stupid comic based on the above