|Andrei Tarkovsky's "Stalker"|
At the Edge of almost every Zone is a cordon sanitaire. If militaries, paramilitaries, or corporations can seal off a Zone, they will. Your best bet in breaking through the cordon is a small, fast, quiet vehicle - and bribery.
There are Free Zones however. These are either in areas that are too geographically remote to seal off and patrol effectively, or are Zones that have gone entirely overlooked. This latter category is also likely to be geographically isolated. And if there isn't an overlay of civilization at the site of the Zone, there may not be enough oddness to the natural environment for a Zone to be detected - certainly not by satellite.
But never fear. This is where drones are your friend. If a Zone is suspected in an area, drones can be deployed to detect them. Once you lose a drone, you know that something is happening. This is how the Institute and a number of corporations scout out Zones in new areas.
There are also "Edgeless Zones". Conventional Zones have an Edge; they are easy to imagine as a set of concentric circles with ever increasing intensity and weirdness as one progresses toward the center of the Zone. In contrast, the boundaries of Edgeless Zones fluctuate so wildly that it is usually difficult to tell where the Zone is at any moment. Imagine an amorphous, constantly shifting amoeba, rather than a series of concentric circles.
Even worse, at least one Edgeless Zone has become unfixed even in relative terms. It's migrating, and has already swallowed a few towns and moved on with a rather unpredictable, saltatory gambol. Behind it lies a path of destruction - and its inexplicable leavings.
Logo courtesy of Hereticwerks.