Expedition One: Into the Mushroom Kingdom
The group moves out down the center, the Marines in formation, while the guides talk about the nature of the game Super Mario Bros. As they travel it becomes apparent that the walls are at obtuse angles from the outer hull, and growing father apart the deeper they go - by the time the group has left sight of the airlock both walls are distant to either side.
The terrain shifts slightly as well, cobblestones becoming interrupted by patches of a fibrous grass - short, uniformly, like a mowed lawn or golf-course. These patches grow until they reach parity, then subsume it until the cobblestone areas are isolated islands within the sea of green.
It's shortly after this - perhaps ten minutes after setting out - that the pipes begin to appear, jutting from patches of cobblestone. Most are perpendicular to the ground, but some are at angles ranging from 45 to 75 degrees. Ahead, beyond the first few pipes, the grassland chamber grows dark... the lights dimming not quite to the point of pitch-blackness, but a noticeable shift that appears to continue into the distance. Some of the distant ceiling lights seem completely dark, or flickering uncertainly.
ooc: It's not an unnatural cut-off or anything, but the light quality will be getting rapidly worse.