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Rondel, known anciently as the Arc of Infinity, was an inter-galactic region of space devoid of all stellar activity. It was the former location of a collapsed Q star, possessed of great quantities of quad magnetism when the star burnt out. It served as the gateway to the dimensions. Omega utilised the Arc as his base, his anti-matter existence shielded by the quad magnetism.
Amsterdam was located on the curve of the Arc when Omega shifted it, Amsterdam being below sea level to maintain pressure for fusion conversion. Using a fusion booster from Gallifrey, Omega planned to become matter again. The Arc later shifted again. (TV: Arc of Infinity)