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. 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") A terrasphere was sometimes seen on the large astrometrics monitor aboard Voyager. (VOY: "The Voyager Conspiracy")