Even years after the first screen appearance many people would remember THE PRISONER as the series with the white balloon.

The white ballon that can be seen during the opening and closing sequences emerging out of the sea and drifting over the shores is called Rover and that's what it was addressed a couple of times. In "The Schizoid Man", an episode which until recently did not play in German, it does even have an important supporting role.
It is only in the first episode of the German version "Arrival" that the thing, for once, is called Hystero. This is actually "Be still!", the command to all Village inhabitants/inmates to stand still without a move. Otherwise they would be in danger of being suffocated by Rover in a horrific image. Here Number 6 receives a demonstration of Rover's abilities.

An underwater test tube seems to Rover's place of birth. On appearing he is recognized acoustically by some impressive dinosaur-roaring noise.

Futuristic and cleverly visionalized, Rover is the absolutely synthesized embodiment of the ever-present oppression in the Village. This watchdog patrols the Village borders, paralizes and catches fugitives. Internally it is utilized to establish and enforce the rules. Quite a lot of pictures show Number Six driven by Rover from behind.

Rovers final realization, though, was rather improvised by the props department.
According to the script of the day of shooting - which is supported by old photographs - Rover was a hemispherical unmanned mechanical vehicle, no windows and a rotating police light on its roof. Thus it was to roam the Portmeirion streets and beaches.

But within the very first days of production in the Village it became obvious that this would not work. The team had imported a vehicle (said to be a VW Beetle frame) which was much too disproportionate and could only be driven on solid ground. Because it was too heavy it used sink into the estuary sand. Elaborate shots were impossible to accomplish - as the dreaded watchdog it was all but ease and unpredictability. Besides, driving proved torture for the person inside, with the sun shining and the way ahead hardly discernible.

Ultimately in shape, Rover is said by some to be McGoohan's idea while others hold it was one of the team: in fact, passing weather balloons must have been the prime source of inspiration. But in front of the camera they wouldn't display their skills. They were too lightweight and tended to vanish in the sky or were easily damaged. So tests were made, they were partly filled with water to improve their bouncing behaviour and to receive a better control in navigating them.
Eventually, what was most important in order to get the best performance, an almost invisible thread was attached to the balloons, they were dragged and the sequence was screened in reverse.

It is this simple explanation why the Villagers had to stand motionlessly while Rover was at large. In the history of movie mistakes and continuity errors there is also one particular scene where, on passing Rover, smoke is seen returning into the chimney - for those watching closely...

...those watching closely will surely discover Rover's "birth" graphically brought to the eye by the Rover mini replica, the famous Lava Lamp. One piece can be seen in Number Six' flat, another one in Number Two's control room where scenes of the Rover coming-to-life can also be spotted occasionally on the big wall screen as a kind of screen-saver.
By choosing this piece of artwork as a prop the PRISONER producers proved their understanding of forms and artistic taste and their fine sense in recognizing contemporary artistic trends.

This designer artwork piece was created in 1963 by one Edward Craven Walker who intoduced it as the "Astro Lamp". In 1965 it was marketed in greater scale as the "Lava-Lamp". Technically it's a glas container with heated up wax in a special liquid forming bubbles which will then rise upwards, cool down, sink and rise again.

There are many sources to be found on the web.