Iain M. Banks last Culture book has a "advanced civilization with more aesthetics than sanity" that had polished a moon much like a marble, that'd then been lowered down towards the planet it orbited while adjusting the orbital speed accordingly. They'd cut a trench around the entire planet, so that said moon were eventually orbiting below the surrounding planet surface.... One of the main civilizations in the book had then "inherited it" and used the moon as the headquarters of one of their military branches.

It's fun to speculate just how bizarre results you could get if you had a civilization with enough resources to do planetary scale arts projects just for the heck of it...

Of course. An advanced civilization with engineering sense wouldn't waste precious building materials on thousands of kilometers of planetary bedrock.

(And that's to say nothing of that area/volume ratio, what that implies for radiative heat rejection!)

“Welcome to Magrathea, where we construct planets made to order.” (Source: The Hitchhiker’s Guide to the Galaxy)