Not every day do you see Metro's iconic ceiling vaults from the other side, but look at this amazing photo.
It shows Metro's Pentagon station under construction in 1974. WMATA recently posted the image to its official Instagram account, to celebrate the 45th anniversary of Pentagon station's construction.
The photo appears to be taken from around the southern tip of the Pentagon building, facing northeast. Were you standing on that spot today, you'd be looking down the middle of the Pentagon's bus station.
It's not clear to me whether this photo shows the actual vault, or rather a mold to construct the real vault over. Maybe someone who knows more about Metro construction can chime in.