An object, usually designed and built by aliens then deserted only to be found by an intrepid exploratory team of humans. Some examples would be Bob Shaw's Orbitsville, or Larry Niven's Ringworld, both of which allow populations in the billions to inhabit a surface area many, many times the surface area of a single planetary surface. Larry Niven's Ringworld, in particular is so huge, that the inhabitants have speciefied away from the base line stock.
Greg Bear's Stone in Eon must qualify as one of the largest BDOs as the seventh chamber doesn't technically have an end point (except, potentially, the end of the Universe itself).
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