The Border Effect
The Border Effect is a term used to describe the unexplained phenomena that prohibits all forms of communications between Surface and the Hinterlands. It is observed to render electronics, including all forms of telecommunications, entirely unusable, as well as inflict a mysterious illness on those who have crossed The Border Effect and managed to return alive.
Throughout time, many have tried to venture towards the Hinterlands, yet very few have ever returned.
Observed effects
In 1913, an expedition group set out north of Slouthceister in order to map out the various coastlines on the other side of Seaport Slouthceister. Communication with the group was lost around 2 km north of Slouthceister's northernmost border. After 7 days, the group returned. While some were very heavily affected by The Border Effect, the least affected stated they had travelled 7 km into unmapped land before returning. They described the terrain as "barren, plain, but surprisingly moderate in climate". At the most northern point they reached, "it felt as if it was physically impossible to move on," and that the "atmosphere felt suffocating".
Cause
Content below is considered metaknowledge.
The Border Effect is actually a by-product of the event that destroyed Mankind's previous world: the aptly named Cataclysm.
The destruction witnessed during the Cataclysm was on a magnitude so colossal that it caused unpredictable collateral damage to the surrounding spacetime itself. The Old World was shattered into several 'fragments'; these can be described as a sort of 'alternate world', an isolated pocket universe that exists in the same space but outside the world's sense of time. To outside observers, it is as if this 'New World' doesn't exist, while the inhabitants of the New World, while oblivious to the existence of the Old World, are still very much affected by it.
Because of the event's unpredictable nature, however, this New World is not a perfect replication of the Old World; instead, its overlap is very unevenly distributed. There are areas with low to no overlap--these are the areas least affected by the Old World--and areas with high overlap, where one could say the 'wall' that keeps the worlds separate is very thin. These areas, which would be described as the Far Lands, act as anchor points of the world where, either through Cosmic Magic or by spacetime fluctuations that cause a 100% overlap, travel between the worlds is possible.
What is described as the Border Effect is simply the observed effects of the varying thickness of the fabric of spacetime dividing the worlds. The increasing overlap between the worlds is experienced to individuals with low magic potency, which would include the majority of Mankind, as reality itself becoming increasingly harder to traverse in; causing a great strain on their bodies, a sort of 'existential pressure' that will eventually cause death. To an observer, it will seem as if the person died without cause.
Should one be able to cross over to the Old World, one would find a desolate planet in ruins. The sky would appear to be in stasis, and remnants from the Cataclysm itself can be seen, seemingly frozen in time. Any remaining life would come in strange, almost incomprehensible forms; amalgamations of previous beasts, corrupted and held prisoner by the Cataclysm. These beings would appear ethereal, shifting in and out of existence, as their existence is partly anchored in the various sub-worlds that share the space.