The Great Exodus

The Great Exodus was a mass-migration even that began in 2601 PLF, following the downfall of the Empire of the Hood. Hood's reign of tyranny had destroyed so much infrastructure on Earth that it had been reduced to a junk world, a husk of its former self, meaning that all of humanity had to evacuated from Earth. Many thousands of ships were used to evacuate humanity from the planet in order to facilitate the cleanup operation, with most of the population being moved to local colonies, most of which were depopulated following the Human-Covenant War and its following conflicts between the Jiralhanae and the Sangheili. Several ships, however, were simply designed to keep the humans in space for extended periods of time, such as the Axiom, the flagship for this particular class.

Another ship that was used to evacuate the population of Earth was the starship UK, which essentially took the entire population of the United Kingdom in one entire ship (aside from Scotland, who apparently got their own ship). The vessel kept Britain in space for several centuries before crash-landing on a quaint little planet, allowing the population of Britain to found the Interstellar British Empire.

Earth was supposed to be cleaned up in the time that the human race was away, but this operation did not go entirely to plan, and for some reason all the robots that were meant to clean up the Earth in this time shut down - rumours that isolated lone units slipped through the net and continued to operate for hundreds of years afterward are, as of yet, unconfirmed or undisclosed.

Whatever the reason, humanity ended up leaving Earth abandoned for nearly 700 years, during which many weird and wondrous lifeforms evolved on the planet - from the vicious Deathclaws and Cazadors to the benign Brahmin Cows. When humanity did eventually return in 3301 PLF, they had to clean the entire planet themselves and destroy the aggresive lifeforms that had inhabited the Earth.