The Memory Arks of the Xerxes were large spacecraft which drifted through space, containing within them the remnants of Xerxes civilisation. Each Ark contained one coffin at its heart which preserved the body of a Xerxes and siphoned their memories to the rest of the Ark. The memories were projected into virtual constructs which were all connected via doorways. Visitors to Memory Arks could walk through these constructs, guided by a virtual avatar of the deceased Xerxes.
The Memory Arks were all linked by a sub-space memoryweb to form one large database of the Xerxes.
One Memory Ark drifted into the Antrozenus Zone and became infested with Thrake. The Eleventh Doctor, Alice Obiefune, and the Sapling boarded the Ark and witnessed a few of its projections (a rocky field home to pterodactyls, a dying red giant, and cylindrical buildings lining the inside of a red canyon) before discovering that the Thrake were eating away at the memories. The Doctor temporarily distracted the Thrake by feeding some of his memories into the tubes coming from the Xerxes coffin. The Doctor led the Thrake directly to the Ark's connection to the sub-space memoryweb, where they all died after overloading on memories. (COMIC: The Memory Feast)