The Silurian Earth (PROSE: Infinite Requiem, Head Games) was an unstable mini-universe, (PROSE: No Future) an alternate timeline/universe/sub-universe where Earth was taken back by the Silurians in 1973. The Seventh Doctor visited, and destroyed, the planet in its respective 1993. (PROSE: Blood Heat)
History
The universe was created by the Monk manipulating Artemis the Chronovore (PROSE: No Future) to meddle in the Doctor's first encounter with the Silurians, which took place in 1973. Instead of regenerating, Artemis made it so that the new Silurian Leader Morka was able to kill the Third Doctor completely after the Silurians captured him, (PROSE: Blood Heat) though the Monk later admitted that he should've known this would only have created an unstable mini-universe. (PROSE: No Future)
The Silurian virus, which came to be called "the Nightmare," spread across the world and killed much of the population since the Doctor wasn't alive to cure it. Some humans regressed to a primitive state, and different dinosaurs that existed millions of years apart co-existed in a new ecosystem that the Silurians began creating. Extinct plants returned and mutated with modern plants, making fruits glow and become inedible; flowers were non-existent and tar pits emerged. The Silurians used atmospheric windows to heat up parts of the planet and had their capital city Ophidian in Africa by the equator. The Silurians built artificial lakes which would house fish to feed a population that was growing as more and more of their people were taken out of suspended animation. They also began to terraform the deserts to bring back the rainforests, returning Earth to its original state.
UNIT was still led by Alistair Gordon Lethbridge-Stewart with the intent of using nuclear missiles against "the Reps," but encounters with the Silurians resulted in the race memory malaise causing humans to behave erratically. Manisha Purkayastha was also never killed in a fire.
Destruction
The Seventh Doctor was later forced to the Silurian Earth, where he considered whether he was supposed to die in 1973 and that he might have existed in an alternate timeline since his third incarnation, but later abandoned the idea, as he had visited crystallized points in Earth's future before encountering the Silurians. Having lost his TARDIS in a tar pit, Ace retrieved the Third Doctor's TARDIS of this timeline and the Doctor materialised it around the Earth, placing the nuclear missiles in temporal grace and deleting them using the TARDIS' architectural controls
As this was an alternative timeline, the changed history of Earth affected the rest of the universe; it subtracted mass-energy from the "real" universe, causing it to end several billion years before it should and about ten million species to never evolve. To prevent this, the Doctor time rammed his "real" TARDIS and channelled the resulting energy to destroy the Silurian Earth. (PROSE: Blood Heat)
Legacy
The Doctor and Bernice Summerfield later wondered if the Silurians and the Garvond (PROSE: The Dimension Riders) were related to their encounter with Huitzilin; (PROSE: The Left-Handed Hummingbird) they ultimately confronted the Monk, and Artemis was freed by Ace. (PROSE: No Future)
Jason and Dr. Who considered "leaving the Silurian Earth to its eventual death" to be one of the Doctor's most heinous crimes. (PROSE: Head Games)