The "Abyss" is the last unexplored place in the world. Strange and wonderful creatures roam within, and it is full of precious relics that present humans can't recreate. Those that dare to explore the depths are known as "Cave Raiders." An orphan girl named Riko lives on the rim. Her dream is to become a Cave Raider like her mother and solve the mysteries of the cave system. One day, Riko starts exploring the caves and discovers a robot who resembles a human boy. 

(Source: Anime News Network, edited)
 
Note: The last episode aired with a runtime of ~48 minutes as opposed to the standard 24 minute long episode.

Episode 1 - The City of the Great Pit

Episode 2 - Resurrection Festival

Episode 3 - Departure

Episode 4 - The Edge of the Abyss

Episode 5 - Incinerator

Episode 6 - Seeker Camp

Episode 7 - The Unmovable Sovereign

Episode 8 - Survival Training

Episode 9 - The Great Fault

Episode 10 - Poison and the Curse

Episode 11 - Nanachi

Episode 12 - The True Nature of the Curse

Episode 13 - The Challengers

Kinema Citrus

Media Factory

AT-X

Sony Music Communications

Sentai Filmworks

Cygames

Made in Abyss