Ido Front

Idofront, also known as the Forward Operating Base (FOB), is located at the bottom of the 5th Layer. It is the deepest a person can go and still survive the curse. It is the place where white whistles begin their Last Dive.

Its main purpose is as a gateway into the sixth layer. It's central tower acts as an elevator through the Sea of Corpses and it can only be activated by the sound of a white whistle.

History
Long ago, it was used as a ritual site by the ancient people that inhabited the abyss. It has since been transformed by Bondrewd into his main base of operations. Here he conducts experiments in warding the curse.

It was here that he brought Nanachi, Mitty, and other children to act as his test subjects, and the place where Nanachi and Mitty transformed into narehate after ascending from the Miniature Garden of Dawn. It is also the place that Prushka was raised as Bondrewd's adopted daughter, before transforming into a Life Reverberating Stone after being packed into a cartridge by Bondrewd himself.

Trivia

 * Based on a comment from Majikajya that Nanachi is a narehate of the "old ritual site," It is possible that the rituals that took place at Idofront long ago involved an individual receiving the blessing. How the blessing was achieved by the ancient people is unknown.