The graphics driver problem is causing the terrain to be rendered incorrectly on certain cards, primarily the 600 series NVIDIA GPUs. This means that the terrain height is inconsistent and you can drive under the terrain as there are abrupt transitions between different terrain areas. Once your ship or SRV passes under the terrain you are essentially in "freefall" where you fall towards the centre of the planet. Presumably this is because the planet still has a gravitational effect modelled but there is no solid terrain to support the ship or SRV. The root cause of the issue, however, is definitely the incorrect terrain rendering originating from the GPU driver issues.
There are occasionally other bugs that can cause terrain problems but what we are discussing here is a very consistent and severe glitch that is readily reproducible with 600 series cards following driver updates that occurred in Oct. When I use my laptop with the 670MX GPU I have severe terrain rendering glitches and these are absent when I use my laptops with the 850M GPU or 1080 GPU. These GPUs are all using the same drivers (417.35 released 12/12/18). Several others have also reported similar issues with the 670 GPUs that resolve when rolling back the drivers. It's a well documented bug that needs a fix either by NVIDIA or FD.