The Great Apothtea was an immense, circular blue sea that was formed in a large crater in the northern hemisphere of Janjur Qom, the San'Shyuum homeworld. The sea was connected to the planet's central continent. The region of Zelfiss bordered the sea.
After the activation of the Halo Array and the reseeding of life by the Forerunner Lifeworkers, a Forerunner keyship crash-landed on the planet, with the resulting impact crater eventually forming the Great Apothtea. Over the following years, the keyship became partly submerged below the surface of the Great Apothtea.
During the War of Wills, the Reformists' fabled fortress of Granduin in the Zelfiss region was undermined by the Stoics and the structure crumbled into the Apothtea. At the end of the civil war, the Reformists boarded the keyship and left behind a massive crater as the dreadnought left the planet. The crater was filled in by the sea.