Terrain problem

Hi folks, i have started my project its going fine but i have run to a problem. I downloaded a car model from Google warehouse which . I then created a terrain the problem is the car drives through the trees and hills in the scene. I have made the car rigdbody but it then falls through the terrain. I'm just wondering have you ever come across this problem as i have searched on the internet every where but not much information came back to me. If you have any solutions that would be of help i would be very grateful.