My tower follows the mouse, but keeps shooting up towards the camera?

I tried to use a Layer Mask on just the Terrain so it wouldn't collide with anything else.

if (hasBeenPlaced == false)
{
Ray ray = Camera.main.ScreenPointToRay(Input.mousePosition);

RaycastHit hit;

if (Physics.Raycast(ray, out hit, myMask))
{
transform.position = hit.point;
}
}
else
{
Destroy(this);
}

if (Input.GetMouseButtonDown(0))
{
hasBeenPlaced = true;
}

 

Probably the ray is hitting the tower each time it's moved.