Fossilised Archs

Who knows what these are or what are their purposes. Of their makers, nothing remains. But there is something to them, a peculiar meaning. Otherwise these monuments would not be visible on multiple dimensions.

Giant fossils

The natural apparition of such a shape would be hard to believe but not impossible, if it was mere rocky archs. But several characteristics indicate that they all come from the same place. Despite being hundreds of meters tall, the archs are made entirely of a single material, organic fossils. They are so old that it is hard to be certain if the original organism was animal or vegetal, but they were extracted from the ground for sure, as fossils can only result of the interaction of skeletons crushed by enormous pressure. However, the vegetal track is prefered, as the existence of animals of this size is deeply terrifying. They may also be the agglutination of multiple fossils, but the archs are so seamless that a minute work using an unknown technology could result in such a clean job.

Another reason is their shape. Every arch has the exact same shape and curve, to the 1000th of degree. Even then, the difference is probably due to the passage of time than mistake at building time. They always go by four, though in some places one or two is missing, collapsed or brought down. Facing the magnetic North, two twins of the same size at the front, the biggest one in the middle and another in-between at the back, slightly out of the axis.

Not alone in the dimensions

The inhabitants of the Dominium thought for a long time that they were the only dimensionfaring civilisation. Most of the other people encountered had a technological level far below theirs, and the few ruins of advanced civilisation like Avalon's Reach seemed to belong to spacefaring folks. The discovery of fossilised archs on now 7 dimensions disrupted this established knowledge. There was at least one other civilisation that once spread across the dimensions, erecting gigantic monuments out of fossilised remains. Although it looks like they have vanished into thin air, as no trace of intelligent life, or even life at all, have ever been found in the dimensions that harbour the Archs.

Gates

A popular theory regarding the Archs says they may be a sort of dimensional portal, much like I.D.E.A.'s gateways. Perhaps they need to be rigorously identical to work, hence why there does not seem to be any connection between the known ones. That does not explain how the builders could reach other dimensions in the first time, but maybe why they are all out of use and it looks like no living being has ever set foot on them. They searched for dimensions filled with life, and ditched those that did not match their requirements.

Entity

Another hypothesis states that the Archs may not be a produce of anyone, but the remains of a once great entity that existed in several planes of existence at the same time. This ubiquitinary creature, at its death, left a part of its body, maybe the ribcage, onto multiple dimensions. Beyond the anthropocentrism of this theory, it is widely disregarded as fantasist mainly because the existence of such an entity would bring chaos to reality as it is known.