The Silurian Earth was an alternate timeline/universe/sub-universe in which Earth was ruled by the Silurians in 1993. It 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.
The Silurian virus, which came to be called "the Nightmare," spread across the world and killed much of the population. Different dinosaurs that existed millions of years apart co-existed in a new ecosystem that was being created by the Silurians. 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.
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 fear causing humans to behave erratically. Manisha Purkayastha was also never killed in a fire.
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. Having lost his TARDIS in a tar pit, Ace retrieved the 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 universe, 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 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)