Descent into the unknown Riko is an orphan living in the city edging the famous so called "Abyss", the limit of mankind knowledge where sience, mith, belief mix so that they can not be distinguished anymore. She is natuarally grown as an junior "cave-raider", moreover, her mother was a very famous adventurer who went missing in the depths, she thinks her mother can be still alive and whish to reunite.
But the abyss is also an extreme dangerous place for humans, and this is the force pushing back, balanced by the strong will of Riko to reach the depths.
In order to follow the entire story you have to watch the Season 1, then the "Dawn of the Deep Soul" and then Season 2.
It is a big adventure, in every sense, the characters are average but this is not a problem as the main player here is really the Abyss itself, the source of troubles, fear, love, discord, pain and glory. Strenghts are the graphic part and the fact that it is varied.
What I didn't like is the ambiguity, how the series looks like from the outside (or sold?): it is a really thought one, violence level is huge but posters shows just 12 years old characters, fluffy bunnies, etc.. so I wouldn't wonder if many children come to watch it by mistake. And it is not just the violence, also some messages are not appropriate for young, when not debatable (especially between the 5th and 6th layer), this time I have to agree with Seoul's certificate of 19 (uncut), definitively not for children.
Near the end of season 2 it starts to twist a lot, lose linearity, I would have appreciated a different conclusion, or that we have some episodes more.