Ur is a postulated supercontinent dating from about 3 billion years ago. "Supercontinent" may mean "all the land above water" but even so, Ur was probably never larger than today's Australia. Ur may never have existed, too, because during the Archean, the Earth's mantle was about 200°C hotter than it is today. The rock formations our lives are built upon couldn't have existed back then. Ur is also postulated to have existed in order to fit the evolution of the planet to certain human-proposed geological cycles. Among the bits and pieces of the theory is the idea that Ur grew and sharnk regularly but existed as a recognizable entity until about 500 million years ago when it was finally broken up.
What evidences the existence of Ur most is today's location of ancient cratons in western Australia, India and Southern Africa. Measurements and calculations indicate these areas were in close proximity about 3 billion years ago during the Archean Eon. Measurements also indicate the southern boundary of Ur to now be below the Indian Ocean beaches of Antarctica, well into the now glaciated regions.
Because of the agglomeration of ancient cratons, Ur could be considered a "supercraton." About 2.5 billion years ago there was a rapid expansion of Ur as more cratons from what are now India, central Africa and Australia were accreted to the edges of the supercontinent. At the same time, the supercontienent Arctica was being formed from the Slave, Wyoming, Superior, Laurentia, Siberia and Baltica cratons
The tectonic cycle of supercontinents coming together and then breaking up has occurred several times since and the last time these regions were in relatively close proximity to each other was during the time of Pangaea, a supercontinent that formed more than 2.5 billion years after Ur.
The largest parts of Ur that survive today are located in India. A supercontinent prior to Ur was possibly Vaalbara while the supercontinent after Ur was possibly Kenorland.