I fucked up my Title. My question is; what's in the 14-17 tunnel?

Secret Dossier, it makes a small icon flash on the screen when you're close to a secret.

It's also totally not worth it unless you're trying to collect everything, to be honest. It only flashes when you know you're near something secret, it doesn't really give you any clues as to how to get to it.

But if you are trying to get everything it's extremely worth it, unless you want to leave a gamefaq up to look at, although I still did...

But the reason I say it's useful is because it will clue you in on some secrets (involving at least 1 gun upgrade, which were super important for me) that no amount of exploration spirit will let you obtain because you basically have to just fall into what seem like bottomless pits for them.

Only thing that was dumb to me was you can't get the dossier till about 1/2 way in or further. And that's assuming you know how to get into the tunnels and how to get at the dossier itself. So by the time you get it it can be kinda worthless.