unity mouse world position
//check for Left mouse Key
if (Input.GetMouseButtonDown(0))
{
RaycastHit hit;
if (Physics.Raycast(Camera.main.ScreenPointToRay(Input.mousePosition), out hit, 100))
{
//hit.point is the position, where the Raycast from the camera
//position towards the ground hit the first collider
Vector3 positionInWorld = hit.point;
//Do something with positionInWorld here...
}
}