Heliopolis was an ancient Egyptian city, a gathering place for scholars and community leaders, also a repository of knowledge, philosophy, astronomy, and the central place of worship for Ra. It is also the name used by Ernest Littlefield to describe the archive of knowledge within the castle ruins on P3X-972, also known as the Library of the Four Races.
From the symbols and the four distinct writings on the walls of the castle, Ernest Littlefield determined that it must have been a meeting place where four alien races gathered to share knowledge or discuss relations. To ensure universal communication, an alien device broke down language into its most basic common components by using a 3D projected representation of the elements of the periodic table. The device depicted 146 unique elements (although only 111 are currently known on Earth) and acted as a Rosetta Stone for a true universal language. The device contained the collaboration of the knowledge of the four alien races of the Ancient Alliance: the Asgard, the Nox, the Furlings, and the Ancients. As described by Daniel, it could be the key to understanding our existence, or "meaning of life stuff." During a violent storm, however, the castle, the device, and the stargate were lost in the ocean, and now only Ernest's and Daniel's notes remain.
Episode Reference: The Torment of Tantalus