Terrasphere

A terrasphere was a type of Undine space station, a large domed structure made from organic materials that included sections recreating class M planet environments. The Undine built these as part of efforts to infiltrate Alpha Quadrant civilizations. The designation was derived from an alternate name for the planet Earth. 

History and specifications
Following the war with the Borg Collective in 2374, in which the self-declared Groundskeepers of fluidic space were defeated with the help of the Federation starship USS Voyager, the Undine leadership established a dozen terraspheres in the galaxy to study humanoid life and train personnel to infiltrate. 

In 2375, Voyager encountered Terrasphere 8 in the Delta Quadrant. This station recreated parts of San Francisco, an Earth city and the site of Starfleet Command and Starfleet Academy. The San Francisco Bay included a replica of the Golden Gate Bridge. Lieutenant Commander Chakotay was secreted onto the station to counter-infiltrate its personnel. He discovered the station and its crew belonged to what was known as Species 4872. After he was captured, Voyager revealed itself to the Undine. Following a tense standoff, [{Captain]] Kathryn Janeway and Groundskeeper negotiated a truce between the Federation and Boothby's faction. After a brief stay at the terrasphere, Voyager departed Terrasphere 8. 

Afterwards, terraspheres were sometimes displayed on a monitor in the astrometrics lab aboard Voyager. 

When the Undine invaded the Alpha and Delta Quadrants in 2376 during a renewed conflict with the Borg, a variety of space stations were deployed in the galaxy. These did not include terraspheres, as the Undine waged open warfare instead of infiltration. 

When personnel learned they had been duplicated aboard Terrasphere 8 after Voyager returned to Earth, they were none to pleased. 

In 2409, the Undine established the Terradome in the Borg-controlled Gamma Orionis sector block of the Beta Quadrant. The Terradome was located in System NGC-4447, within the Alini sector. This terrasphere recreated part of Deep Space 9's promenade, including Quark's Bar. In that year, Starfleet and the Klingon Defense Force dispatched a five ship strong task force to investigate activity in the system. The task force clashed with the Terradome's defenses, which included a replica starship, the USS Poseidon (NCC-8472-U), and eventually rescued the Human Akira Sulu from among a squadron of imposters. 

The USS Cerberus (NX-95560) was one of the Allied starships visiting the systems. 

Later that year, the Federation dispatched the powerful telepath Celestara Mex to fluidic space to establish diplomatic contact with the Undine. Her ship, the SS, was escorted by a Starfleet vessel but captured by the Undine. Its crew killed, Mex was held captive in the Undine Base, a terrasphere attached to a "land mass". Mex guided Starfleet telepathically. After dispatching the terrasphere's defenses, an away team boarded the habitable area, which replicated a class M surface littered with organic structures native to fluidic space. Following the rescue of Mex, the team found an Iconian gateway hidden in the habitat. With the diplomatic overture a failure, the Federation ship returned to the Kuda system.