@PogueSquadron There is some weird part there where the way you go back is actually different than the way you came in, since you have the tank now. Just go as far back as you can and once you can't go any longer (since it's too small for the tank to fit through), fly up and you'll find a warp that takes you back near the beginning of 7 or something?
ALSO... don't forget this when you're done exploring and you want to get back to 8 from 7. I was stuck there for awhile like how the F do I get my tank back down now? because it doesn't come down like it did with the boss fight the first time. Then I went online and realized a lot of other people get stuck there too, totally forgetting that (like myself) they left by that new route and should come back by that new route as well, which is to say just take the new warp near the beginning of 7 to get to the door to 8, don't travel all the way back to where you fought the boss because you can't get through that way anymore.