The underground skyscraper is a multi-storey building that extends deep underground. Each floor would be a walk-out floor opening out onto an underground promenade. For efficient use of space, the promenade would be centrally located with units facing either side of it.
What would be best is a multi-storey
underground galleria, containing an entire neighborhood in vertically stacked form, and all the relevant facilities associated with one.
This kind of setup could either be located in the heart of a city, where real estate is at a premium. Or else it could be located in some remote region with an extreme outdoor climate, so that the underground area would serve as enclosed climate-controlled habitation shielded from the external environment/elements.