Made in Abyss

The Abyss is the world's last unexplored region, a vast cave system filled with strange creatures and valuable relics that modern humans cannot reproduce. Explorers who descend into it are known as Cave Raiders. Riko, an orphan who lives on the rim, dreams of following in her mother's footsteps by becoming a Cave Raider and uncovering the Abyss's mysteries. During one of her explorations, she discovers Reg, a robot who looks like a human boy. Their meeting gives Riko a new companion as her fascination with the unknown depths begins to turn into a much larger journey.