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.