I read the manga and just finished watching the anime.
The anime gets to chapter 26 of the manga(sans last scene of it), the manga is at chapter 42 now.
It did a pretty good adaptation job, some filler is added, but not much.(Though some see it as causing others to hate the female lead, I think it is in turn showing us the female lead can do more than just talking.)
Warning, don't start the series if you cannot accept the following:
1) nudity of children
2) tragic, extremely tragic story
3) Psychopathic adopter
4) Violence, gruesome
5) Nanachi the fluffy bunny(?) stuff toy girl
and if you really want to avoid people mistaking you as a pedophile by simply judging from the drawing.(NOT a good series to read in the public or in front of your parents)
The series' characters are cute, but the series can get really gruesome. Seriously.
Genre: Wikipedia labelled it as Fantasy Adventure, I'd say at least from chapter 27 onward, you can definitely add "Horror" in the mix.
Somewhere in the world, people found a shaft of about 1km in diameter opening to the underground, Abyss, the last unknown place in the world. It goes at least 20km into the ground.
While Abyss contains powerful artifacts of the past, it is dangerous.
Plot: Wikipedia is a good start. The story starts with the female lead Riko finding the male lead Reg, who is a Robot(artifact from the Abyss) in the first layer of Abyss.
Personally, I like the fantastic world of Abyss and the characters are very deeply illustrated. The artwork is done in extreme detail, even the more draft like parts can capture you into the scene.
Characters have more realistic personalities, and not stereo types.
The mangaka went through deep efforts to show you what life means and how you cannot turn back. The Abyss will curse all who go up in it, especially humans, most can still return in the upper layers, but only experienced raiders can return from the third layer and only a handful is allowed in the the 5th layer. Once you get to the 6th layer, it is considered a one way journey, you don't necessarily die from the return trip but you basically lose your humanity. From the 7th layer on, certain death. So you can only press on.
1st layer: dizziness, vomit
2nd: series vomit, headache, numbness
3rd: on top of the above, strangeness in the sense of balance, dellusion and hearing voices
4th: pain in the whole body, blood coming out from all holes
5th: losing all senses, confusion, self injuring.
6th: loss of humanity, probability of death
7th: certain death
The main characters makes friends and some of them just die in the arc or the next. If Nanachi didn't greatly increased sales, she'd be dead much earlier.(You can see multiple death flag set up for her death)
Personally I like Ozen the unmovable, she acts all cruel, but is actually very caring. The mangaka stated he drew inspiration from mixing minotaur and witch so you get a strong, towering, but hunchback lady which gives an uncanny smile from time to time. But she is cool and cute at the same time.