In version 3.4 beta 39, it's finally possible to visit underground levels by setting a negative elevation. The minimum elevation of the point of view depends on the current minimum elevation of levels.
To avoid quality problems, I finally limited the max elevation to 100m.

The minimum elevation is set to -10m at this time. If you find a good reason why I should allow a smaller value, please tell me before I release version 3.4 (it should happen soon).