Interstellar space stations have been discussed recently.
I was reminded of a story I read recently by Arthur C. Clark story called Rendezvous with Rama. Essentially a monolithic cylinder comes charging into our solar system one day, from the deep depths of space. This cylinder, 60 kilometers long, 16 wide, contains inside of it what seem to be cities, infrastructure, advanded technology and an ocean the shape of a gigantic ring.
Alien urbanism, floating through deep space.