Easter egg: Go to the "languages" menu. Highlight any language you want and hit right once to see a symbol appear on the space ships. There will be a short clip of a man doing a stunt.
Easter egg: Go to the second page of the "special features" menu. Enter the "cast and crew" menu. Once here, highlight any person and hit right to make a symbol appear on the picture of the city. This is a short clip of men doing stunts.
Easter egg: Go to the second page of the "special features" menu. Once there, highlight any one of the special features and press right to make a symbol appear on the pyramid. There will be about a ten second clip of behind the scenes footage.
Easter egg: Go to the first page of the "special features" menu. Once there, highlight any one of the special features on the page and press right once to see a short clip of how the filmming crew put on make-up.
Chosen answer: It is residual radiation from atomic weapons used in an attempt to fight off the Psychlos.