The Avenite Crystals of Aalthir

Avenite is a very rare mineral, crystal clear like glass but much lighter, almost weightless. It is found on the Elemental Plane of Air and also occasionally on cloud islands on which cloud and storm giants build castles. Named by a Thyatian alchemist wizard after the old Thyatian name for bird, Aves, because it enabled constructs to fly like a bird. 

Avenite can be enchanted to levitate when a Fly spell is cast upon it, and it can carry vastly more than its own volume, perhaps a hundred times. It also responds well to Permenance spells, so if the Fly spell is made permanent, the crystal will levitate for decades, perhaps centuries. This has proved incredibly useful in creating flying structures, including some Alphatian skyships (though not all use Avenite), flying building and even flying cities. 

It is massive Avenite girders running through the underbelly of Aalthir that keep it afloat.  Without them supporting the city structure, Aalthir's million tons of rock, earth, buildings, goods and people would crash down to the surface, killing everyone in the city and causing devestation for miles around. These Avenite girders were installed underneath the city before it ever lifted off, the girders were then enchanted and then the rock underneath was removed to detach the city from the ground. 

Avenite, as one can guess, is a very valuable mineral, and therefore worth stealing. It is possible to remove the Avenite from the foundations of Aalthir though if enough is taken this of course will affect the buouyancy and stability of the floating city. 

The illegal miners are a gang from the less salubrious quarter of Aalthir where non-mages live, including manual workers, dwarves, halflings and a few actual thieves. They chip away at the crystals that then fly up to the top of the tunnel in which they are digging. The crystals are prised off the ceiling and into a bag of holding where they can be carried around with neither upward forces nor bulk - bags of holding are ideal for smuggling the Avenite crystals out via the Hall of Portals. Customers for these fenced crystals include mages and engineers from across the Known World, especially Thyatis and Glantri, Alphatia's major rivals, who have no qualms about an Alphatian city falling to its doom. 

Here is a quick map of the tunnels into the Avenite crystal girders. As can be seen, the mine starts with a ledge protruding from the side of Aalthir's rocky base from which the thieves have worked inwards. The small pool of water is where the miners accidentally hit a drainage pipe - the water is from the streets  above and is dirty and smelly. 

This could be modified and extended for a far more complex network of tunnels and chambers. Also the stats for the miners and their guards is left to the DM so the encounters can fit the level of the player characters.

It is unlikely any wild monsters will have found their way into the tunnels as the entrance is a mile above the ground, and half the time it is over seas and oceans. Nonetheless it is possible for flying creatures that nest in caves to find their way in - giant bats, stirges and harpies spring to mind. It is also possible that the thieves or maybe their overseers have brought along or summoned monstrous guardians such as constructs, animated undead or elementals.