Hi ghaaliove,
I see that you are fairly new, so I welcome you to the FPSC forums.
If this is your first attempt at making a map then you may be confused as to your earth.
The ground you see below you is the bottom of the skybox.
The ground your building would set on are actually segments that you need to place.
All this is covered in the manual (included PDF file), and the
manual is a must read for any newcomer.
Make sure you read it right away.
If you are familiar with the way the ground works and this is a custom model you have made then the offsety is off.
I think its the offsety, but I'm not sure.
Try using the Forum Search Box at the bootom of this page.
Usually it is the quickest way to find your answer and saves forum space by not posting the same questions.
Hope this helped