A terrasphere was a type of large space station built by Species 8472. The bulk of the station consisted of a domed chamber in which holographic technology and particle synthesis were used to create a full-scale replica of Starfleet Headquarters and the surrounding area of San Francisco on Earth. There were over a dozen terraspheres scattered throughout the Delta Quadrant.
Terraspheres were heavily fortified, protected by an unknown type of force field and shields and armed with sophisticated weaponry. They were powered by thirteen thermionic generators located beneath the surface of the simulated city.
Species 8472 constructed the terraspheres following their hostile encounter with the USS Voyager in 2374. Fearful of Voyager's modified nanoprobe technology, they used the terraspheres as training facilities in preparation for a mission to infiltrate Earth and determine Starfleet's intentions and capabilities. Inside, members of Species 8472 genetically modified to appear as Alpha Quadrant species practiced conducting themselves as Earth residents.
In 2375, Voyager discovered Terrasphere 8 by tracking what appeared to be a Federation subspace transmission. Captain Kathryn Janeway was able to negotiate a peaceful settlement with the terrasphere's leaders. (VOY: "In the Flesh")