Skyfall

Sometimes on remarkably rare occasions, a City of Blue falls from the sky and collides with the ground below. The name stock after a British soldier stationed in Ireland during a rebellion proclaimed "The sky was falling" while broadcasting a radio signal to his superiors. The only words that can make out from the broadcast due to interference what's sky and fall. The term Skyfall made its way through the British Military and naturally everywhere else. It is a term that is never taken lightly as the soldier who unknowingly coined the term died during such an event.

Consequences

When a blue City falls to the ground the results can vary. Sometimes it isn't instantaneous cataclysm that can be felt countries away such as that of Huratula, the city that hovered over Ireland. When it hit the ground the cast out of roar and rumble in a flash of light that was reported as far away as New York City.
The city above Ireland decimated the nation save for the Erin Isles and a select a few coastal regions. It left a crater in the land that used to belong in that crater hovers in a ring around the nation, The Erin Reef.

No Man's Land

There have only been 8 skyfalls recorded since the blue cities first emerged and every one of them could have been postponed or completely avoided. Anytime Skyfall happens it changes the world in a very permanent way.
In the southern U.S. a menagerie fell, and while it didn't explode in such a dramatic fashion as it did in Ireland, it did have consequences all the same. Before, the city was distant and the beasts within kept away from the defenseless people below. Upon impact there was no way to restrain the creatures that poured from its impossible architecture.
The states that would be Oklahoma Texas Louisiana and Colorado became infested with these creatures who adapted to their new habitats well. This area is referred to as No Man's Land and is seen as a frontier of sorts for explorers.

What causes Skyfall?

The cities of blue are seemingly alive, some are even capable of communicating with those who enter. Some believe that this consciousness, if it is indeed conscious, can die, and if it does the city can no longer function. Some cities don't die at all. Several cities are incapable of producing and harvesting the Aether that powers them. These cities rely on harvesters to replenish their fuel. If they run out before a harvester can get to them in phase either where it needs to be within the city, skyfall is inevitable.

Response

The initial reaction to skyfall is shock. It's humbling to witness the raw power that these events display.
What comes after is a drive of opportunity. Looters come from far and wide to take advantage.

They salvage all they can, hunting the creatures that escape and selling their parts, ripping the technology from the rubble in hopes of fetching a fair price, and killing any competition that crosses their path. Many areas have yet to be fully explored and picked clean.

The Emerald Crown

The Erin Reef is a product of skyfall, a series of floating island that border the broken emerald isle. The reef and all of Ireland are a place where physics doesn't behave as it should.

The survivors, referring to themselves as The Exiles of Erin , often called The House of Exiles, reside in the reef and in the broken land it surrounds. They were the first victims of skyfall and resent The Cerulean for what has happened to their home.

A Tragic Sight

A skyfall is always a tragedy. It may not the obvious in the moment, but over a long enough period of time, the consequences of skyfall will be made clear. Sometimes its a complete change of the land, a tear in reality, or a surge in the population of monsters on the surface. Regardless it leads to the inevitable loss of life.
Some organizations have dedicated themselves to studying the way the cities move including their velocity and direction of movement. Their goal is to predict skyfalls before they occur. By analyzing which Harvesters fuel which cities they can usually predict when a city is at risk. The major issue is the prediction of how much fuel city has to begin with and how much is deposited by a harvester. This makes their jobs difficult as it isn't an exact science.
On occasion these observers have noticed the complete abandonment of cities my harvesters that used to do dutifully refuel them. This occurs for numerous reasons. Should have Harvester be looted of its aether, it will be put behind schedule and sometimes fails to compensate for lost time, prioritizing more "vital" cities. Sometimes it happens without reason or warning. The harvesters simply stop coming.