Akihito Tsukushi: 6 books

In an age when the corners of the world have been scoured for their secrets, only one place remains unexplored--a massive cave system known as the Abyss, filled with monstrous creatures and lost relics. Those who delve into its depths are known as Cave Raiders. A young orphan named Riko dreams of...

Reg and Riko's descent to the bottom of the netherworld has begun! With a perilous journey ahead and their pursuers from the surface not far behind, the trek is already testing the limits of their strength. Will their quest end before it's barely even started?

Riko and Reg have left the Seeker Camp and ventured further into the Abyss! The mysteries and peril that await them are the stuff of nightmares--but these nightmares are ruthless, deadly and very real. Do they have the courage and strength to survive as they descend into the deeper layers?

Riko is finally on her feet again after her near-death brush with the orb-piercer. But the threat of death is always close in the Abyss. The Garden of the Flowers of Fortitude isn’t far off, and its splendor hides a perilous secret!

The trio have arrived at Ido Front! The depth’s sixth layer is almost within reach, but getting there is easier said than done. Before they can continue their journey, they have to get past the Sovereign of Dawn: Bondrewd!

Riko and her friends have finally made it to the sixth layer! Not long after taking their last dive, they find themselves in a village populated by Hollows who possess a peculiar sense of value. Mysteries, terrors, and bewildering truths await them in the Abyss' deepest depths!