Near the border of one of the great cities one unfortunate enough may encounter one of their border patrol units. Their task is quite simple, prevent anyone from getting inside or out of the city. This task usually is done by outright killing the trespassers or escapees, but not always. Those who are not killed are however never seen again, as they are taken to one of the several underground facilities situated near the borders, where experiments are carried out for the good of everyone else. Of course, these patrol units are rumored to be a result of some of said experiments so if the experiments really bear fruit for the good of civilization is up for debate.
The green liquid that is used to sustain these and other creatures of their ilk is something that was discovered long ago.
Oh and this was also my first drawing that I did solely on a surface 4!