A punishment zone.
Punishment zones were areas on Rubicun III where criminal violations resulted in execution. The zones were designated randomly each day as a means of discouraging any criminal actions and the location of the zones was kept secret from the Edo citizens.
The random and secret location of the zones was intended to serve as a deterrent by creating the ever-present possibility of execution for even the smallest crimes. Across the planet there was only one punishment zone per day and justice for any crime committed within the zone was administered by the Mediators. (TNG: "Justice")