The Way Back is a tunnel that leads to the Planet. It has 16 levels with a break between parts 7 and 8.
There is a hidden level in the tunnel. When you beat The Way Back, part 7, it will show out a cutscene featuring the Angel, Bunny, Dulicator, Child and Pastafarian sitting on the Boat, navigating through the large gap between part 7 and 8 (though this gap is significantly smaller than the one between B-tunnel and N-tunnel). During the cutscene, you can see the boat navigating through the tunnel with a yellow color. But as the Duplicator (and the Child's balloon) fall out of the Boat, landing on the yellow floor, you are immediately skipped towards part 9, with a purple colored floor.
The Part 8 of this tunnel can't be played when you press on the Galaxy Map. You can only play it by using the previous/next selection button once you reach part 9 of the later part of the tunnel.
The Way Back, part 8 is the only "hidden" level and the one you can navigate through by sitting on a boat. You can see the Child's balloon floating in space too.
The following is a brief description of the story in The Way Back. More information on the story can be found here.
There is a rejected level called "id-286", looking a lot like part 2. Originally gonna be placed between parts 4 and 5.
The 15th level was designed by mathwiz100, in the Topaz Set (Surprise!).