"Ground is a continuous plane and a stable reference point. It is the surface on which the conflicts of urban propinquity: public and private, planned and impromptu, privileged and disadvantaged, are worked out. In cities like New York, great cultural significance is placed in being on the spatial ground [...] The ground plane remains a reference point for cultural life. Hong Kong enhances three-dimensional connectivity to such a degree that it eliminates reference to the ground altogether. Hong Kong is a city without ground." A. Frampton, J. D. Solomon, C. Wong in Cities Without Ground.
The longest outdoor covered escalator system in the world, making one feel more like inside a large urban interior, rather than being outside on the streets: