Why are there giant concrete tunnels in the desert?

You are saying that it takes 8 minutes for light to reach earth but we’re not sure that’s true as we have not accurately measured the speed of light. We have measured light after it bounced off a mirrored surface to a receiver where the laser was emitted from. We don’t know if the speed of light is the speed of light to the mirror and Instant on the way back or so on so we don’t know that light travels as fast as gravity or faster then gravity.