5th Layer

The Sea of Corpses (なきがらの) is the fifth layer of the Abyss. It is a vast frozen wasteland encircling a massive sea. It is also the last layer that it is physically possible for Delvers to return to the surface without losing their humanity or dying.

Description
The 5th Layer is the world that only White Whistle or Black Whistle Delvers (sources contradict; see ) delve into, and according to Shiggy, one can count the number of people who successfully returned alive with the fingers of one's hands. Vertically, it is the shortest layer of the Abyss at only 1,000 meters in depth, but it is also the widest horizontally, estimated around ten times the size of Orth. The most prominent physical feature of this layer is a large sea inhabited by primeval creatures and littered with bones, hence the name “Sea of Corpses”. Below the regular water, a layer of thick, slimy water suspends the sea in place. At the sea's central point is a large waterfall, providing access for Mail Balloons to ascend from below. Large structures formed of crystalized water, known to delvers as “frost”, are interspersed throughout the sea. The structures themselves are referred to either as “water supports” or “water bastions”, the latter being permanent formations, which are held up by the Force Field.

As the boundary between the 5th Layer and the 6th Layer begins just below the water, much of the aquatic portion of the Sea of Corpses actually exists within the 6th Layer, with the 5th Layer playing host to the sea's surface, Ido Front, and the surrounding lands. Since it is the last section of the Abyss where it is physically possible to survive a return trip, it is quite common for White Whistle Delvers to settle down here for some time.

At a depth of 13,000 meters, the deepest point above the water, a forward operating base named Ido Front was established by the White Whistle Bondrewd. Ido Front was originally the ruins of a former ritual site. Ido Front gets its electricity from the rotation of the unstoppable whirlpool it is built upon. A device akin to an elevator exists in its center, the Altar of the Absolute Boundary, that is used to transport people through the sea and into the 6th Layer. However, it can only be activated by a White Whistle, or more specifically, by a Life Reverberating Stone. Since Life Reverberating Stones are specially designed to be exclusively used by their original owners, the elevator is only available to those who have a White Whistle of their own.

In this layer, the Curse takes the form of extreme sensory deprivation, first draining one of their basic senses. Riko herself does not notice that she had fallen down a flight of stairs. The Curse then quickly escalates into losing one's very sense of self (medically, it is referred to as proprioception). This kind of deprivation also makes unintentional self-harm extremely likely, as seen when Riko clenched her jaw so hard that she broke her own teeth, not realizing the harm done until she put her finger in her mouth and saw blood.

Species

 * Hamashirama
 * Meinastilim
 * Narehate
 * Stingerhead
 * Unknown Creatures

Flora

 * Eternal Fortunes

Individuals

 * Bondrewd
 * Failed Umbra Hands
 * Gueira
 * Human-looking silhouette (Sightings)
 * Ilim
 * Meinya
 * Prushka
 * Umbra Hands

Artifacts

 * Altar of the Absolute Boundary
 * Canopy Unto Dawn
 * Far Caress
 * Gangway
 * Your Worth
 * Shaker
 * Sparagmos
 * Third Works
 * Zoaholic

Points of Interest

 * Ido Front
 * Altar of the Absolute Boundary