unity3d – Translating mouse position to world position yields constant result

I am building a topdown 3d shooter. I want to find the position of the mouse in the world so I can shoot a bullet from the player’s position towards the mouse position on mouse click. So to begin with I’m trying to get the positions of both objects:

void Update()
        {
                if (Input.GetMouseButtonDown(0))
Debug.Log(
      "Player position = " + transform.position + 
    "; Mouse position = " + Input.mousePosition + 
    "; Mouse position in world = " + mainCamera.ScreenToWorldPoint(Input.mousePosition));
        }

These are the results:

Clicking right on top of the player
Player position = (0.0, 0.5, 0.0);
Mouse position = (968.3, 543.1, 0.0);
Mouse position in world = (0.0, 25.0, 0.0)

Above the player
Player position = (0.0, 0.5, 0.0);
Mouse position = (957.9, 686.7, 0.0);
Mouse position in world = (0.0, 25.0, 0.0)

Right of the player
Player position = (0.0, 0.5, 0.0);
Mouse position = (1114.0, 557.7, 0.0);
Mouse position in world = (0.0, 25.0, 0.0)

Below the player
Player position = (0.0, 0.5, 0.0);
Mouse position = (962.1, 414.1, 0.0);
Mouse position in world = (0.0, 25.0, 0.0)

Left of the player
Player position = (0.0, 0.5, 0.0);
Mouse position = (822.7, 557.7, 0.0);
Mouse position in world = (0.0, 25.0, 0.0)

So looks like the translation to world coordinates always yields the same result?