The Mariner's Valley is the longest-inhabited and most densely-populated region of Mars. Before the Earth Origin theory gained favor among modern historians, it was thought to be the cradle of humanity. Its steep cliffs retain a thick, oxygen-rich atmosphere in the rich valleys below, while shielding them from the brutal dust storms which once constrained the inhabitants of Xanthe and Araby to cling to life in craters and caves.
While the continual growth of cyanobacteric algae in the northern ocean has gradually thickened the atmosphere and made life feasible at higher and higher altitudes, the Valley remained the economic center of the northwestern Martian world up through the 11th century. This made control of the Straits of Capri - the sole conventional entrance to the Valley - the most valuable geopolitical prize on the planet.
In the late 7th century, the Province of Xanthe had led the western coalition to a smashing victory on the fields of Osuga against the County of Fadoua, ending its Governor's attempt to unify the province of South Margaritifer and seize control of the Straits. Xanthe's continued aggressive expansion in the decades which followed, both into the crater confederations of West Xanthe and the coastal lands along the Straits raised fear across Western Mars that it, too, would make a play to become a regional hegemon.
The peripheral powers which formed the Copratian League to oppose Xanthe's ambitions - the canyon confederation of Ius and the newly-unified and rapidy-growing Province of Kasei - faced a critical geographic problem. Separated from each other by Xanthe, they faced the fear that a Xanthean fleet which could cut communication through Da Vinci Bay could strike a critical blow to one of the allies before the other even learned that a war had begun.
A solution came from an unlikely source - the then-desolate county of Echus, which had recently turned to Kasei for protection against the steppe hetmans which plagued its river settlements. Towns in its southern periphery had contact and limited trade with the hermit kingdom of Hebes, from which it was separated by 12 kilometers of the highland 'Death Zone' - lands so high that the air was too thin for life to survive permanently. Specially-acclimated people who were reared at extreme heights could climb up the Echus cliffs and survive the passage to the other side. Hebes folk legends claimed ancient heroes had once done the same across the 70 kilometer gap between southern Hebes and the Marinerian county of Ophir.
This hope, thin as it was, was enough for Kasei to try. Countless bold and hardy highlanders perished in subsequent attempts to recreate legend - but eventually, it was done, with the help of sealed goatskins full of low-altitude air and the discovery of a midway chasm deep enough to allow rest. Using similar methods, adventurers were later able to find a second, shorter route through the county of Nia via the uninhabited Hydrae Chasm to the source of the Maja river in the isolated county of Juventae. The now-famous Hebesian and Hydrean Runs were open.
For the first generation of the 8th century, mail runners regularly carried secret dispatches across both routes, keeping the allies connected during several brutal wars with the Xanthean Caprisian League. After the 724 defeat of the Copratian fleet at Batoka Bay, these paths became the only form of communication between the Valley and the outside world.
Ultimately, they were not enough. Caprisian forces finally subdued Ius in the bloody battle of Calyport, while Kasei was first forced to cede the rich craterlands which had once been its heartland, then collapsed under invasion from the province of Tempe and the resurgent plains hetmanates. For its part, Xanthe was able to enjoy the fruits of victory for only thirty-odd years before a disputed gubernatorial succession led to civil war and conflict with the new leading Marinerian power of Melas.
Knowledge of the Highland Runs was lost for generations until evidence was rediscovered by archivists in the mid-12th century. Today, with a much thicker atmosphere than existed in the 7th century, the two Runs are popular among hikers and marathoners who see them as an opportunity to relive history - providing a much-needed boon to the struggling economies of Hebes and the north Marinerian states.
