Thread:ProjectPredacon/@comment-26464372-20150320202214/@comment-25958301-20150324215351

They left a small portion of Reach untouched, wanting to extract that Forerunner artifact/crystal that warps time and space, meaning they reached locations (like to Reach) from 3 hours to just 20 minutes.

And the Covenant are very precise about the extermination of mankind. They literally cover every square inch of a planet. Nothing's left. And the glassing beams (or in the Halo Books, bombardments of plasma rather then beams) cover wide areas if I recall, using a crisscross grid patern to get every inch. .3.