I rated it 4 because it sucks when they are underground at the beginning and mainly because there's no fucking end or decent explanation. I'm kinda new to mangas but I'm already sick and tired of mangas with no ending or explanations. I've read like 10 mangas and 9 of them are like that.
The story is ok, it's about earthquake, tsunami, volcano, like any "dystopian" manga story is. The female, as always, wanna save even the monsters. 70% of the story is about "fear" and "darkness" (apparently these are the only words they can think of) The other 30% is the main character saying "daddy mummy sissy" like it isn't obvious they are all dead since the first chapter. Philosophy bullshit that get's old after the first volume and keeps being forced forever. The army people with big guns can't handle the crazy kids with spears, saying "they are too well armed". The heli pilot had a bazooka for NOTHING.
And as always, no ending or decent explanations. Almost zero gore, horror or proper explicit violence (even tho I found this while searching for gore, violence, horror, disturbing content). Cheers on wasting my time. I was really hooked, even with all the bullshit philosophy, but the lack of ending ruined everything.
Worse "ending" than this, only every single manga by Junji Ito (the guy is simply unable to explain stuff).
But after all this rant, still, this manga is the best I've read so far. That's how low the quality of disturbing mangas have been since I started reading it.