The Sea of Corpses is the 5th layer of The Abyss. A vast, frozen wasteland on top of a massive sea, it is the last layer where it is physically possible for a delver to return to the surface.
The 5th layer is the world that only White Whistles delve into and according to Shiggy, you can count the number of people who successfully returned alive with the fingers of your hands. It is the thinnest vertical layer of the Abyss, being only 1000 meters deep, but it is also the widest one horizontally, probably around ten times wider than the town of Orth. It consists mostly of a large sea with some crystallized sections held up by a large layer of thick mud and bones, hence the name "The Sea of Corpses." The water is filled with monsters of the sea, and diving deep into the water is synonymous with suicide. Not only are there dangerous creatures but swimming back up would make someone succumb to the curse quite easily. 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 Whistles to settle down here for some time.
At the deepest point above the water, a Forward Operation Base named Idofront was established by the White Whistle Bondrewd. Idofront was originally a building used by the ancients as a ritual and prayer site. The FOB gets its electricity from the rotation of the unstoppable whirlpool it is built upon. A device akin to an elevator exists on its center tower 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.
Here, the curse takes the form of extreme sensory deprivation, first draining one of their basic senses to the point of Riko not noticing falling down a flight of stairs. The curse then quickly escalates into losing one's very sense of self (medically, it's called proprioception). This kind of deprivation also makes unintentional self-harm extremely likely, as 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.