Collision is messed up on terrain

I made a terrain, and loaded a raw file that I made in world machine, then I decided to smooth it out in unity, and the collision is messed up. when I go around with my player it makes the collision like it was before the smooth,
so basically i fall through ground and stop after 20m, i walk go up hill and stuff but it does not match the actual look of the terrain.

How do I fix that?

A quick search suggests there is a known bug in Unity 5, requiring you to save the scene in order to update terrain collision mesh: Manually Update Terrain Collider - Questions & Answers - Unity Discussions