[vidarh]

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...

[nitrogen]

Wouldn't the planet's atmosphere and oceans fall into the trench?

[CanSpice]

Yes, which is why you dig the trench deep enough to cover it over again, make it airtight, and pump out all of the atmosphere to make a vacuum torus inside the planet.

[drakeandrews]

There were either walls on either side of the trench, or the planet was a barren rock to begin with.