Terrasphere 8 was one of about a dozen environmental domes, known as terraspheres, built by Species 8472 throughout the Delta Quadrant to train its people for eventual infiltration of Earth and the Federation. The facility contained an elaborate simulation of the Starfleet Headquarters in San Francisco, Earth, and was occupied by the fluidic space lifeforms isomorphically altered to live as humanoids.
Powered by thirteen thermionic generators, the simulation used a combination of holographic projection and particle synthesis. The recreation included not only much of Starfleet Headquarters, but part of the surrounding city of San Francisco, including the Quantum Cafe (although the simulation did not extend to Market Street). It was never determined exactly how Species 8472 gathered its intelligence.
These facilities were used for training exercises, beginning in 2374; this was shortly after the USS Voyager's first encounter with the fluidic space beings, and continued for about a year until peace overtures from Captain Janeway – in the form of technological exchanges – persuaded them to cease their activities and return to their realm. (VOY: "In the Flesh")