It was the very first close-up on the US's infamous secret base - Area 51 (aka Groom Lake) using a super duper 1140 mm lens to capture all the HD glory. The series showed satellite photos that revealed new buildings, runways, and towers. There was also unmarked planes that conducted "secret flights" between Area 51 and Las Vegas.
Youtube user integrajoey has the complete episode (divided into 5 parts), so check that out if you don't have History Channel at home.
And the related stuff, a papercraft of an Area 51 sign post that you'll find around the site's perimeters.