The 3D basemaps are currently not designed for underground visualization. For example, the building layer contains open data with or without information about underground levels. The elevation mode relative to ground and the extension of a building into the ground is added to assure no gabs are visible between any given ground and the building.
However, this is a great use case! Thank you for illustrating how you want to use the 3D basemap for underground visualization. We are considering to add this capability in the future.
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