The D'Arsay archive was a spaceborne data-storage facility launched by the D'Arsay over 87 million years prior to the 24th century. Made of fortanium and other minerals, it contained records of artifacts and thousands of personalities from myths within the D'Arsay culture. It had the capability of scanning foreign computer systems and imparting its data while re-programming it to exhibit its records. It also had a tractor beam.
After launch, it spent 87 million years drifting in space for over two sectors and eventually was located in sector 1156. It had accumulated minerals over time, completely obscuring its structure and making it appear as a comet.
The USS Enterprise-D discovered the comet in 2370. After scanning the starship, it began to use the replicators on board to change parts of the vessel into different mythical settings contained within the records in the archive. These settings were from a drama involving Masaka, a sun goddess, and involved locations such as an aqueduct, a temple, and a swamp.
The archive also altered Data's programming, giving him personalities of key mythical figures, such as a terrified D'Arsay boy, a person named Ihat, an elderly man who portrayed Masaka's father, a victim of Masaka, and Masaka herself. Furthermore, the program turned the internal mechanism of a photon torpedo into snakes and turned engineering into an inferno. Captain Jean-Luc Picard sought to remedy this by playing along in the myth, using the persona of Korgano, another mythical figure, to persuade Masaka, and in turn the archive, to restore Data as well as the Enterprise-D to their normal states.
According to science consultant André Bormanis, the explanation given in the original script was that the archive was an "advanced Genesis Device" that mistook the Enterprise for a world on which to recreate the ancient society. (Star Trek: The Next Generation Companion (2nd ed., p. 285))
The archive was one of the first CGI models, but not the first – as there were digital starship models previously constructed for evaluation purposes (see: here) – , of a space-bound artificial structure in the Star Trek franchise. It was however, the first one not realized as a physical studio model, but rather directly as a fully textured CGI model, and as such, the very first one actually seen by the general Star Trek audience. Based upon a foot-high sculpture Dan Curry made, Santa Barbara Studios (SBS) constructed a CGI model. "I considered doing a drawing of the library but finally opted for a 3D approach that allowed me to determine how it would look from different angles. The art director Richard James wanted the architecture to have a hint of Mayan look so I created that by sculpting it from wooden blocks that I cut together in the shop of Image G. Ron Moore flew it up to Santa Barbara Studios and they translated it into the CGI model of the library that is revealed when the comet starts to melt.", Curry revealed. (Cinefantastique, Vol. 25, No.6/Vol. 26, p. 58) The studio also provided the comet and "steam" effects in the episode when the archive was freed from its icy enclosure by the Enterprise-D's phaser fire. Curry has retained the mock-up sculpture as part of his personal collection. (TNG Season 6 DVD-special feature, "Departmental Briefing, Year Six: Dan Curry Profile")
When the remastered version of the episode came along in 2014, it turned out that the computer files for the archive most likely did not exist anymore, partly because SBS – who had constructed their model in their own proprietary Dynamation software – was no longer in existence, partly because it was discovered that SBS had not been in the habit to archive their digital models.  The visual effects company responsible for the remastering project, CBS Digital, was left with no choice other than to faithfully recreate the structure in their own software, LightWave 3D, as featured at the top of this entry.