Nav Mesh problem~ I need help!

I'm currently work on a assignment in creating a racing game. I used nav mesh to let the ai avoid obstacle and follow the track. Diagram 1.0 show how my terratin top view look like. The highlight blue color is my baked nav mesh.

Diagram 1.0

I designed my track by heighten my terrain and set lower part as lava/water as visual effects. Diagram 2.0 show the perspective view of my terrain.

Diagram 2.0

But after i set the script to let my ai car go to a destination, my car drop lower from the ground. Diagram 3.0 is where my ai car placed in the beginning and the ai car become like what shows in Diagram 4.0.

Diagram 3.0

Diagram 4.0

I wonder what is the problem? Is that because of my destination y position? Is that any way to let the nav mesh agent ignore the y position of the destination but only targeting the x and z position? Hopefully i can get solution asap. Urgently :(