Terrasphere

Terraspheres were a series of simulators built by Species 8472 in 2374, designed to mimic conditions of Starfleet Headquarters on Earth.

Overall, there were approximately a dozen terraspheres scattered throughout the Delta Quadrant.
The polymorphed members of Species 8472 used these spheres as training grounds, and were conducting a role-playing mission to better understand their enemy, the Federation and Starfleet.

Terraspheres were heavily fortified, and were protected by an unknown type of force field and shields. In all, they were powered by thirteen thermionic generators located beneath the surface of the simulated city.
The bulk of the station consisted of a domed chamber in which holographic technology and particle synthesis were used to train its people for eventual infiltration of Earth and the Federation.

The USS Voyager tracked what was believed to be a Federation subspace transmission, which led to the discovery of Terrasphere #8.