Ancientmind was one of the first graveminds ever in existence. He is on the forerunner city "Neon" which is on its way to the planet "Ceres".
Ancientmind has been as big as he could on "Neon" for the past 1,200,000 years. After the forerunners fled, "Ancientmind" has been chasing them with dispersal pods, Infection colonies and even Forge. If "Ancientmind" had more room to grow, he would be planet size by this time.