Grey Matter Series Volume Three: Secrets of the World of the Unknown The third installment of the series as Mark Trogmyer ventures into the World of the Unknown. This time Mark uncovers various secrets of his past, and present within Earth, the World of the Unknown, and the Spirit Realm, which shines some light into how his future could be. Mark also learns that things aren't always what they seem to be, as his life is going to be in very real danger. With sightings and stories of Nerogroben's return and a dark shadow that has been hiding within the dark Grey Matter Clouds resembling Mark himself, will Mark be able to uncover the truth before his untimely demise?