Discoloration on the level

The project our group is currently working on is having some problems. For some reason, “parts” of the terrain have different colors, or appear to have more shadows. Though, some parts of the level doesn’t seems to be affected at all and appear to be fine.
We can’t figure out what the problem is at all, it might be problems with the terrain or the lighting.

We would really appreciate all the help we can get.
Here’s some examples of our problem: