Apparently, it's the Pentagon! See below, from www.dcmilitary.com
. Do a search on Google.com for "hell's bottom" and you'll get lots of info.
"The Pentagon groundbreaking took place Sept. 11, 1941. Three shifts worked 24 hours a day, every day, building the Pentagon wedge by wedge. Some 1,000 architects worked in a hangar at Hoover Airport producing blueprints to stay ahead of the 14,000 construction workers.
The building displaced an airport, a racetrack, factories and a low-income neighborhood known as Hell's Bottom. The idea for five sides came from the original location at Arlington Farms which was bordered by five roadways."