We think of the ground as something stable and permanent. When we are between buildings, it is the firm plane that we all most commonly navigate on. We consider the ground to be a natural, perpetual, and unchanging presence.
Architects and designers also think of it in another way, as grade. Grade is the shape of the ground, and architects are particularly interested in where it meets their buildings. Grade is artificial. It is shaped to meet our needs—mainly to help us navigate from place to place, building to building.
A recent Episode of 99% Invisible discusses this issue in some detail. It describes how downtown San Franscisco is built up on a substrate of gold rush era ships. It talks about creating new land in Dubai and Singapore, as well as the issue of illegal sand mining. Also, it discusses the archaeosphere, the geographic layer of the earth the we ourselves have created over time.