Utopia windows were a technology created by a race that were extinct millions of years before the Last Great Time War. They created virtual parallel universes. It was impossible to die within one.

Because of the Time War, many Utopia windows were spread across the cosmos as flotsam and jetsam. The Ninth Doctor accidentally landed inside a Utopia window, believing it to be a parallel universe where the German Empire won World War I. The Doctor was later joined by Winston Churchill, who travelled through the window in 1942. (AUDIO: I Was Churchill's Double)

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