Watch now, or you would never know

Trains are not the only things that travel through train tunnels. In fact, light travels too. Have you ever wondered how much further you can see the light coming from the other side of the tunnel? I have found a very convincing clip that explains a lot in silence.

Man! You are ruing my experiment!!
Yeah, we still have the Sun. Take care guys!
(They must be belonging to an unknown subterranean civilization.)

5km

Another 5km