Its the Shield World of Onyx. if you listen to Guilty Spark closely, he mentions a "Shield Instalation" in one of the levels, i forget which one, might be "The Ark".

The shield world itself is where the the 4th Halo novel "Ghost of Onyx" took place. The world itself is composed entirely of metal, and had some pretty intriguing secrets. I hope the continue the storyline from here.