Let's start with the good things though, and the first one is the entire first 3 episodes. They are everything you could want from the introduction to an adventure anime, the monster attack is thrilling, the setting is fascinating, the characters are fun, there's that one scene with the great animation and music that makes everyone melt, they're just great.
In addition to the first 3 episodes, the conclusion is great. They make episode 13 twice as long, introduce a totally new storyline, put the characters in an impossible situation that rips my heart to shreds, then sends us off with another gorgeous cinematic display.
That said, for how good the beginning and end are, the middle of this show is a spectacle, just not in a good way. Starting in episode 4, it's almost offensively boring at times, the plot never feels like it's going anywhere at all, and when it does, they retroactively invalid all the progression, the sense of danger the Abyss had is crushed through multiple red herrings that don't actually matter, the great music and animation all but disappear, and I started to realize I just might hate the characters, particularly the main protagonist.
The main character is a girl named Riko who fits the standard "optimistic protagonist" mold we often see in adventure stories to keep things moving, the problem is she is one of the worst examples of this character I have ever seen. She's not so much optimistic as just stupid, she is perfectly fine with the idea the Abyss is just going to kill her in a way no mentally healthy child possibly could be, at no point in the series does she ever display any pre-emptive fear at what she will most likely be facing, and it makes no sense whatsoever. It's made even worse because she's played off to be extremely knowledgeable about the Abyss, most likely so the author has an easy way to dump setting exposition, but in that way, the writer's need to flesh out this admittedly fascinating world directly compromises the plot and characters that inhabit the world. Lastly, the author of this manga is very obviously a huge pervert into no shortage of fetishes that left me personally completely disgusted. There are multiple scenes with the 12 year old main character needlessly naked. There are scenes of her pissing herself. There is a scene that is not called "torture porn" for nothing (to be fair, that scene did have actual impact and brought made Abyss feel threatening again). There's a scene where Riko wets the bed while sleeping with Reg. There's a scene where she gets trapped in a giant carnivorous plant's stomach. And there's a scene she makes Reg take her clothes off then makes fun of him for getting an erection before the show makes the innocent act of Reg performing mouth to mouth to save her life creepy by choosing then to tell her he'd kissed her. When you stop and think about it, this show is disgusting with its sexualization of minors, and it does not apologize for it in the slightest, the only redeeming quality is that at no point does it present any of the said minors being taken advantage of, but that's hardly an accomplishment unless the author wants to oust himself as a pedophile.
All in all, the flaws of the show don't take away from its great moments, but the great moments don't justify the flaws either. This is a hard anime to pin a number to because of that, but I kind of have to, and since at the end of the day I liked the good slightly more than I hated the bad, it gets a 6
