Clarke ring

A Clarke ring is a megastructure that surrounds a planet, essentially a giant ring in geostationary orbit, usually linked with the main planet by three or more pylons. The less massive and faster-rotating the planet, the smaller the ring. The Terran Ring and the Martian Ring were the first such constructs built by humans, though the soronen, renlai and First most certainly built such things earlier, none have survived completely intact.

The most difficult part is constructing the pylons - particularly on massive worlds like Earth. The Foundation Arcologies only began the issue - they had to move with the plate tectonics of their host world, in addition to anchoring the ring with flimsy material. Without hadronium, this can be impossible in some cases - the tensile strength needed rises above - or even close to - that of carbon monofiber. The Terran ring had the distinct advantage of mobius patterns, and is the largest known structure to be made with such.