The Way Back is a tunnel that leads to the Planet. It has 16 levels with a break between parts 7 and 8.
This tunnel can be accessed by completing Box Storage Area, part 11, and completing the Going Home Checklist. There is a hidden level in the tunnel. When you beat The Way Back, part 7, you unlock Level X, in which the Angel and the Pastafarian find a floating boat. You then have to click The Way Back, part 8 on the Galaxy Map, and it will show a cutscene featuring the Angel, Bunny, Duplicator, 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.
The tunnel starts when the Angel has gathered all the people that want to go home. After part 7, a cutscene is displayed, where the Duplicator and the Child stand looking at the gap between part 7 and 8. Then Level X is unlocked, and upon completing and clicking on part 8, a boat thing comes, and they jump in and are taken across the gap. The Duplicator falls out, then Level 9 of The Way Back starts.
The 15th level was designed by mathwiz100, in the Topaz Set (Surprise!).