Tet-Herumopet is an area of the Abyss notorious for being difficult to navigate, full of tightly packed clusters of islets. It is most known for being the first place where leviathans were "seen". The myth was born here.
Tet-Herumopet is part of what is called the "outer rim" of the Abyss. It's a bit of a misnomer as the outer reaches of Kykr civilisation constantly expands, so the outer rim will not be outer for too many years before the ABN has extended beyond the previous rim.
What it means, at present day, is an area that extends between 35 and 60 hours journey from Dunia and very poorly covered by abyssian bouys. It is, however, littered with islets.
The treasure trove
What makes the cluster so gloriously deadly is also what makes it so attractive. It is densely packed with islets, floating freely and chaotically all around the area. Some islets are as close as several hundred meters apart at times, and with their chaotic movement and wildly varying sizes, the cluster is a nightmare to navigate.
What these islets represent to those that are able to exploit them is raw resources. The islets are packed with valuable minerals and organic compounds that can be mined and extracted by those that are skilled and knowledgeable to do so. One of the primary resources being extracted here is Thallium-203, a vital metal used in fine electronics after refinement into Thallium-204.
Myth and legend
In spite of its endless promises of treasure, there is of course that little detail of the leviathan myth. While the myth of megafauna monsters roaming the darkness is permeated all throughout the Abyss, nowhere is it as popular as when you're talking about the THC. In fact, the myth itself was born here, with some expeditions getting lost without a trace and submarines suffering unexplained, catastrophic damage from "something large".
Those less superstitious explain this with islet collisions, which is far more likely than giant monsters that no one can find. But, alas, the Kykr are a superstitious people, so naturally they cling to the myth like glue.