From Halopedia, the Halo wiki
Along with Green Hills, Second Base was one of two colonies glassed by the Covenant directly after Harvest in the Human-Covenant War. Initially, the United Nations Space Command (UNSC) had hoped to cover up their loss by explaining it away as a communications blackout caused by rebels. Eventually, they revealed that the UNSC was now fighting a xenocidal alien hegemony, and that Second Base and Green Hills had both been destroyed with the total loss of two million civilians between them.
In January 2530, the Assembly conservatively estimated that it would take somewhere between one hundred and ten years to three hundred years to re-terraform Second Base after the Covenant glassed the world.
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