There are Holes in Reality opening all around Night Vale, blending between reality and some other versions of reality, which are relatively similar to reality but slightly different. Most holes appear in the sky, but many open in houses, the street, the public library and even in Night Vale Community Radio Station toilets.
The holes are opening as a result of the fractures in reality, which were caused by Huntokar when she tried to remove Night Vale from the world in 1983, but they have suddenly widened due to the presence of the 5-headed dragons which attacked the city, as well as the presence of so many gods.
The only known way to close these holes and prevent them of reappearing is by acknowledging truths that distinct their reality, such as the existence of angels, or the abnormality of a town being run by literal monsters.
Effect of the Holes
The holes allow things to pass between them, and essentially they let the different realities they connect to flow through them. People near holes remember things that never happened, and forget other things that certainly did. Sometimes only the people of one of the realities can see the people from the other one. Although most of those who experience such transitions between the realities forget it, some remains are often left in the area when the hole was opened. The holes also cause people to know things they did not know before. For example, after a hole was opened on May 22nd in the skies, Cecil knew for a brief period that Michigan is a state in the US, and that its capital is Lansing, according to the Community Calendar. A short time afterwards, he unknew all of that. The holes also release a lot of extinct dinosaurs, mostly pteranodons.
A connection between the realities was presumably possible before the holes began to appear around town, or else the holes existed for a long time in a small quantity, because Cecil found a cassette he did not remember recording, but he saw his younger self from the Other Reality recording in the radio station bathroom. In addition, in the cassette Cecil mentioned a brother he does not have, but came visiting him through a hole in reality in "Cal".
It is said by Huntokar, when she broadcasted her story on the radio, that some realities might fall into each other, and that the holes are the first stage in the process.
The Other Realities
The Other Realities to which the holes lead are mostly quite similar to The Reality, but have a few major differences. As Huntokar says, an infinite number of worlds exists, some of them are very alike and others are extremely different. There is a reality where Night Vale is an enormous metropolis, and one where it was never founded. There is a Night Vale exactly like my Night Vale, but in which on a single day, a single citizen wore a green shirt instead of a yellow shirt. All of them were hurt when Huntokar tries to remove her Night Vale off the world, and shattered the fragile existence instead.
There is a world where humans never came to be. There was a Night Vale in which Old Woman Josie would never die, one in which she had never lived. There was a Night Vale in a world that had flooded, and the town thrived on an island surrounded by dark, deep water.
A nuclear war, presumably the one Huntokar tried to protect Night Vale from, did occur eventually (in 1983) in at least one universe, as seen in one specific reality that often connects to ours. In one specific reality that often connects to ours, Cecil never came out or even met Carlos, and he knows about Michigan. He has a brother named Cal, who suffers from radiation symptoms, and no sister. In addition, in the other reality there is a need to come out, even though in Night Vale people don't care about others' sexual preferences.
Mayor Dana Cardinal's father never died of cancer in another reality, and she never killed her double, either. In the Other Reality, the dragons never conquered City Hall or destroyed town, and there is no public library or Recreation Center. Larry Leroy, out on the edge of town, is married to Chrysette in the other world, a woman he never met or knew. In addition, some Other Reality appears to have a lot of pteranodons, as they are flying out of some of the holes. A reality in which cars are made of cheese exists as well. No one knows why. There is probably some symbolism to this.
Cause of the Holes
In 1983, when her world was on the brink of nuclear war, Huntokar, the goddess of Night Vale, tried to do the undone and to isolate her beloved town from the world. The result was a rift through all other Night Vales in every other world, making some versions of the city to fall into each other, while other versions balanced precariously on their broken reality. Recently, due to the massive presence of gods and dragons in Night Vale, the fractures in reality are widening and the rift between realities deepens. This causes more and more holes to appear, causing the different realities to merge slowly, until all worlds will collide and presumably destroy each other.
Shutting the Holes
Cecil says that people of Night Vale tore the realities apart themselves by not acknowledging the truth about them and their world. Legalizing the acknowledgement of angels, the departure of the dragons, Alondra's decision to let the angels inherit most of her mother's estate, the Vague, Yet Menacing, Government Agency resuming the routine of spying on people, the librarians returning to the Night Vale Public Library, the City Council coming back from their holidays, Dana Cardinal remembering that her father died, people admitting that many things that happen in Night Vale would be considered as not normal in any other place and accepting the reality of their world healed the broken reality. The acknowledgement of things that are part of the day-to-day life in Night Vale, as things that do not happen anywhere else helps distinguish The Reality from other realities, and prevent them of merging. As for August 2017, the residents of Night Vale, despite acknowledging the town's abnormalities, did not manage to prevent all the holes from appearing or the realities from colliding, but the did manage to mitigate the effect and shut down several holes.