Step 1: Make the Long Tunnel

First make the tunnel out of something dark like black wool or coal blocks

Step 2: The Glowstone

Then put glow stone on the start of your tunnel then put glow stone every 10 to 20 blocks then it will look very dark when your walking along

Step 3: Inside and Seeing What Its Like

So when you go inside the tunnel you see grass on the floor get rid of it and put the floor as the same block as you made your tunnel with
So now I will be showing some pics of what it looks like when you download a map like this and your walking through a tunnel like this.