Visualize a layer underground/pitch a map underground

Motivation

First of all thank you for your awesome work at Mapbox! I love building with your platform!

Now, the feature request:
In urban planning/smart city visualization (among other areas) it is necessary to visualize data underground. For example pipes, drainage basins or subways should be visible when displaying a city and planning for it, as else valuable information might get lost. Here, it would be great to have the option to visualize such data under and not above the ground. Or preferably even pitch the map underground.

Design

1. The currently experimental z-offset of layers should extend to negative values
2. If possible an easy "glassview" toggle configuration property should be available to make all layers at/above ground level transparent.
3. If possible the pitch (and/or zoom) property should extend beyond zero to view the layers from 1 in detail. I am aware that this might be more difficult with the current map

Mock-Up

There is a good example for ESRI's sceneview how this could look like. Of course with the nicer Mapbox models and styling.

Help welcome

In the meantime I would be very grateful for any hints on how too implement this using the current Mapbox state!

No files, tests, or specific entry points are identified. Start by tracing the experimental layer z-offset and map pitch/zoom handling in the rendering code. Done would require a defined, tested approach for negative layer offsets and underground viewing, with any feasible glassview behavior documented.