User blog comment:Argorrath/Treyarch posts a message of zombies; possibly hinting to a DLC 4/@comment-71.7.166.132-20130731193315/@comment-211.30.125.8-20130801084830

Agathara (in the Zombie Story) is the core of the Earth. In Buried, if you do Dr Maxis Side, He trys to go to Agathara so he can destroy the Earth from the core and reunite with Samantha. (This is from the quotes from Buried when you do the EE.)