Historian's nightmare

Historian's nightmare is a colloquial name for an unsolved problem in the field of world history. Specifically, it refers to the odd fact that many ruins that can be found across Laurasia, belonging to cultures that should (at least according to the history as we know it) not be possible.

Proto-Laurasian explanation

The usual way historians attempt to solve this issue is by pointing to Proto-Laurasian empire, an ancient empire rumoured to be the predecessor to all known empires today. The problem with this interpretation, however, is that (according to the sources) the architecture and technology is pretty distinctly more advanced than that known to the current civilizations. This tends to be explained by theory of devolution, the idea that this proto-empire devolved due internal conflicts (or other means) and progressively fractured into empires we know and love today.

While there is no direct evidence against this theory, the lack of link between the ruins and the actual existence of proto-empire means the theory is on shaky grounds.

Technological advancement

The ruins mentioned, apart from being in the wrong location, also have other strange properties. Usually, they contain items, contraptions or are built using principles that are not known to the given civilization or the civilization simply does not have resources to build them.

Examples might include (but are not limited to) dwarven paths scattered and uncovered far beneath the surface of Laurasia, so deep that the special structures that kept them from collapsing are unknown to dwarfs to this day! Or orcish temples and the trinkets found within possessing magic that can be barely explained by state-of-the-art magics known to Elves today.

Missing civilizations

Ruins of this kind of almost all civilizations can be found around Laurasia. However, there are two notable exceptions.

First exception are Elves. While one can occasionally find Elven ruins in unorthodox places, often with dubious origins, it is obvious that this are not advanced in the sense the other ruins are.

Second (and complete) exception are human empires. All ruins that have been found so far (of human settlements that is) can be traced back or at least theorized to have some concrete origin. This by itself should not be that surprising. Humans are known to be one of the youngest races to form empires of note, and thus any settlements and structures will likely be documented by Elves (or other civilization).