An interesting concept, orphans are trained to dig up archeological relics, I assume so they can pay for the upkeep of the orphanage. One of them, Riko, is obsessed with going to the bottom of the abyss, where the relics are, she wants to find her mother also who has definitely gone to the bottom. The girl finds out that her mother might still be alive, and that serves to doulble her desire to find her mother.
The story starts a bit slow, we are introduced to their world, and some of her fellow orphans. Then out of the blue a mysterious robot boy appears as Riko is being chased by one of the many monsters in the abyss. She takes him to the orphanage after he passes out, and while he is unconscious studies him. They become friends and are soon on their way to explore the abyss. Many adventures ensue, and they meet various humans and creatures, as they explore the various levels of the abyss. They have some troubles along the way, but through perseverance and sheer luck they survive.
The main character, Riko, is so fascinated by the wonders of the abyss that she is oblivious to the dangers she faces, but the robot boy, Reg, is there to save her. That is something that bothers me about this anime, Reg is there to keep saving her, because she seems to have no idea of what dangers are there. She has information and has studied but has no field experience, she is constantly distracted and loses herself so thoroughly that she puts herself in danger constantly, if Reg wasn't there, Riko would be dead a thousand times over.
Each level of the abyss has its own uniqueness, and the scenery changes, and they are drawn fairly well. The different levels have their own creatures, plants and dangers, and as they go deeper things get more difficult.
The story is good, the animation is good, the scenery is drawn well, but the characters are pretty basic. The sound is fine too. There are a couple of scenes that make no sense and I have no idea why they are there, just a couple of things that have no explanation. The story progresses a bit too fast after they enter the abyss, seems like they wanted to cover each level with just one episode. The next to the last episode is when things get really interesting, and in the first half of the last episode some things get resolved, just not everything though, there is a lot of unresolved and unexplained things left at the end. It's like they were in a hurry to finish it, and put some things in there that would satisfy most viewers, for me it was a bit disappointing. I hope there is actually a second season, I'm sure those of us who enjoyed watching this would like to see a completion of the journey.