Mapzen's map-tile servers are haunted. If you use Tangrim maps tonight you will find that they have been attacked by a ghostly reanimated fog. The map also appears to be showing the locations of the normally hidden poltergeists of the world.
The fog background seems to be have been created by using some foggy background images, which have been animated by continually adjusting their scale and opacity. You can use the search box at the top of the map to zoom to a number of worldwide cities.