The Sisters Grimm study the occurence of spontaneous portals, and have been monitoring and mapping them for years. They say that until Alex and Conner were born (which changed the speed of the timelines between the two worlds), roughly once a month, a portal would open for a few seconds. Sometimes, a person or creature would pass from one world to the other by accident, and the Sisters then tried to capture them and sent them back with the next portal. Occasionally, a creature got away and caused "sightings" in the Otherworld.
The pattern of the portals, when drawn on a map, is that of a perfect spiral around the world. It pinpoints to the next portal, which the Sisters predict will stay open for much longer than a few seconds. It will occur in New York City (see: New York City Portal).
The portals lead from somewhere in the land of Stories to various places in the Otherworld: the Sisters mention influences of magic in all ancient civilizations and claim all mythical creatures known today once existed in the Land of Stories and went through the portals.