A couple of ideas inspired by the sidebar on First Age wondrous infrastructure in Arms:
The Coiled Forge
The Cinder Isles are known for their volcanoes, and the Exalted of the First Age did not hesitate to find uses for them. The Coiled Forge was carved out of one that had gone dormant, into the shape of a serpentine dragon coiled around itself, its head raised over its body and its maw wide open. Great shards of obsidian were mined and placed along it as scales, four massive spikes of red jade made its fangs, and when the geomancy was complete the volcano was dormant no more. Ferrous lava continuously dribbled from its mouth, tracing a path down the curve of its coiled body. As it reached the plain, men and magma-beasts toiled at a great ironworks that both separated the iron ore from the lava and then used the remaining heat to smelt it. Further down the plain, it would be beaten into blackened sheets, then rolled on cylinders of cooled igneous stone to the ocean. Here, finally, they would be lapped and riveted into warships as deadly as they were ugly. Spheres of pumice, with their centers bound to never quite forget the heat of their origin, would be loaded into the holds of these ships and launched from deck-catapults. It was a fleet of these worships that shattered the shining towers of Chiaroscuro during the Usurpation.
Now, the forge lies buried under its own cooled lava-stone, the geomancy mercifully disrupted and the volcano dormant once more. It could be excavated and repaired, but if the volcano became active before everything were cleared, the entire mountain could explode.
The Great Current
In the Age of Sorrows the Great Current is a rapid, unceasing current cycling around the Blessed Isle. Maintained by the storm and ocean spirits according to the Ten Thousand Oaths, it is readily visible to any sailor by the lighter-hued color of its water. It is an integral part of the Realm's infrastructure, but it is a poor shadow of its lost namesake.
In the First Age, ships were hardly necessary for carrying most goods. If you wanted to transport bales of Eastern spider-silk from Deheleshen to Randan, you simply packed them into watertight barrels, leaving enough air in so that they could float. You then hired a scribe to prepare a document describing the contents of the barrels, with apologies, praises, prayers, tolls, bribes etc. to the appropriate spirits, and - dumped the barrels in the ocean. Within a few weeks or months your goods would arrive at their destination, none the worse for wear.
Because of this convenience, much of the ancient knowledge of sailors was effectively lost, and had to be rediscovered anew in these lesser, fallen ages.
Broken scraps of this system still exist, and stories of fishermen's daughters finding love letters written to one of their ancestors fifty generations past are, on occasion, true.
