Game object floating in the air

I can't seem to get my game object to stop floating in the air.
When I add a rigibody to it all it does is fall through the ground

 

Add rigidbody and make sure you have collider of some sort on the ground. See collision rules in the manual: https://docs.unity3d.com/Manual/CollidersOverview.html

 

If it falls through the ground he has a collider(@Lurking-Ninja edit: didnt see you said on the ground.), the problem is likely mesh on mesh, if they are both not convex(concave) they will ignore each other.

Pro tip - check out the docs and manual.