I should say this first and foremost: beyond chapter 30, this is no longer for the faint of heart. The writer is hellbent on making you as uncomfortable as possible when reading and by god does he do a brilliant job at it, and not in a "uhh i shouldn't be reading this" uncomfortable.

Story: 10
Due to the excellent worldbuilding and the fact that the author has hammered home that basically anything is possible in the Abyss, nothing feels forced or deus-ex-machina. The driving force for the main cast is easy to understand and easy to root for, and the obstacles they face along the way genuinely do feel like walls instead of speedbumps (that is a major issue I find with other stories, it never feels like what the protag is facing acts as a genuine threat). Combine this with mini-arcs that hit an uncomfortable nerve (most notably the arc from ~ch40-current), the world around them feels truly hostile and serves as the obstacle.
These uncomfortable arcs feel well integrated into the story, and the author does not include them for shock value or to make the story "edgy" and "gritty". It serves to show the harsh reality of life in the Abyss, and it does so excellently.

Art: 8
I don't have much to say about the art. It's consistent, it looks nice, and it does its job well. It doesn't come anywhere near Murata's level or anything, but I can't think of any faults in it either.

Characters: 9
This would have been a 10 if Riko and Reg were not children; I find that adults have more complex reasons and reasoning that allows for more meaningful interactions between each other and the environment. As it stands however, children being the main cast (apart from Nanachi) felt like the right choice, since it highlights the divide between their competence and the harshness of the Abyss. Them being children also helps so that we instead of see the world through youthful optimism and clarity, and opposed to the cynical professionalism an adult would have had to adopt (i.e., Ozen).
The author perfectly captures the innocence of youth without making either Riko or Reg feel too naive. When it comes down to it, they can both grit their teeth and soldier on, even if they lack technical experience. I would really like to touch on Riko's character, especially how she treats Reg, but that would go into spoiler territory; I would have also liked to touch on Nanachi, but you future readers will have to see that for yourself.

Enjoyment: 10
The characters' fleshed out personalities and clear drives are a breath of fresh air compared to many stories I've seen recently. The worldbuilding is elaborate without piling on too much exposition or unnecessary detail, and the way it poses as a threat to the characters feels very real, in that it is neither forced nor laughable.

Overall: 9.5, though I will have to round it down to 9.
Just go read it.