I'm trying to do some mod work in Megaton (more specifically adding another building), and need to see the ground so I know when the building touches the ground (so the building isn't floating in mid-air). I've tried generating terrain in the World LOD menu, but this doesn't resolve the issue.

Any help or suggestions?

I'm not experienced with any aspect of Exterior editing/modding. The only work I've ever done is Interior.

Not 100% sure... do you just see water or similar in exterior cells? How about people? You might try hitting F5 to see if it refreshes. This usually fixes issues where you can usually see the ground/npcs/etc. fine, but for whatever reason they go invisible.

If it has always been the case, is it only from a distant zoom? If so, check under options/preferences or what not (sorry, don't have access to toolset ATM) and check for view distance and increase that.

If it is neither of these and you don't figure it out, let us know and hopefully someone will be able to help with more details.

Go into view > show/hide window and try checking/unchecking the 'terrain' option.