Look, there's a lot of stuff in this bunker, but what's this doing here?
It's an indoor tennis court! It's a huge building, with the ceiling being way above them, as expected. The place has whites taking up the exposed ceiling design and the courts being blue. There are windows towards the sides of each court, and a door to enter the rooms behind the windows, letting you spectate a game if needed, but that's not really your concern right now.
There's around 4 tennis courts here. They seem about standard size, and they seem to be well-maintained... at least you can't seem to find any scruff marks or the like. Baskets filled with tennis balls are all over the place, and rackets are on the walls. There's even tennis ball launchers!
Towards the far left hand side is another set of doors - that's probably your way out. That's neat, but something else also captures your attention - a little machine in the middle of the room. Looking at it closely allows you to see that it's "Takimura Technology's Adjustable Tennis Courts" - if the controls weren't locked right now, you would be able to change how the tennis court looked to a variety of scenes, like an outdoor tennis court or one in the sky.
Huh, I wonder who this was for.
(edited)