(SPOILERS ALERT!) Of its aesthetics (especially the landscapes and character design), at first it looks like a children's series, but it ends up suddenly and unexpectedly transforming into a cruel and almost sadistic anime.

The initial premise is quite original: on a remote island there is a huge abyss of unknown depths. Inside that gigantic peak it is possible to find objects of forgotten and unknown origin, the so-called relics, which attract adventurers and seekers of fame and fortune from all over the world. Among them are the Cave Raiders, true explorers of this underworld who will not hesitate to descend thousands of kilometers despite the risks involved.

The story focuses on a girl, Riko, who is preparing to become the best Cave Raider. During an expedition into the abyss, Riko encounters a humanoid robot she calls Reg. Together they plan to escape to the bottom of the abyss to meet Riko's mother, a prestigious Cave Raider who has recently disappeared and is presumed dead. The Abyss presents a very complex structure divided into a series of levels with different characteristics and ecosystems, inhabited by a variety of creatures. Not to forget the mysterious "curse of the abyss", an evil that affects those who try to climb back up from the greatest depths, causing pain, dizziness, vomiting, bleeding, loss of humanity and even death. The hardest part of this season, was that of Nanachi and Mitty, it comes past the first grim plot twists, this reminded me of one of the most traumatic stories I've seen in an anime since Nina's in Fullmetal Alchemist, with which it has some common ground. This brutal contrast, between the comical moments before which it is impossible to hold back a smile, and the cruelty and sadness of other scenes before which it is impossible to hold back some tears :(. About the characters, Riko is the best protagonist this series could have: an intelligent and self-sufficient girl, incredibly vital, with an enormous willpower, a contagious optimism and an amazing charisma. And Reg is, almost always, a good support, besides supposing by himself another of the mysteries of the Abyss that arouse so much interest. Otherwise, the role of the rest of the characters is somewhat brief in these first 13 episodes, although all are well differentiated. Some of the most representative are the sinister Ozen, whose appearances are most tense; the endearing Nanachi, one of the surprises of the series; and Lyza, the mother Riko. Positive aspects: The originality of the idea from which it starts, the way in which the interest of the abyss is always maintained. The designs of the creatures, the animation, the soundtrack. The contrasts between the pleasant moments. Negative aspect: The disconcerting semi-sexual scenes that Reg brings with Riko are completely superfluous.