CoreWorld is a hollow ball at the center of the Earth, about one thousand miles in diameter, giving it an internal 'equator' some three thousand miles around. Gravity pulls outward, against the walls. Therefore, there is no 'ceiling'.
Forests, deserts, cities, etc rise up and overhead in eerie, otherworldy perspective. In other words, a city fifteen hundred miles away would appear to be directly overhead, a thousand miles straight up.
CoreWorld was inhabited by a human tribe of CoreWorldians who believed that the stories of people living on the surface of the Earth were only legend. Also common were pterodactyl like creatures called Pturbodactyls and spiders. A race of stone Golem was kept in check by the continous light that shone from the CoreSun.
However, when Prime Evil came to CoreWorld, he used a Dark Jewel to extinguish the CoreSun. The Golem were able to transform into sentient beings and being grateful to Prime Evil, agreed to serve him and capture all the human CoreWorldians and form them into an army of slaves.
Prime Evil had also stolen a Megalaser from the future and planned to use it to drill straight up towards various strategic places on the surface world, capture the worlds leaders and take it over for Evil himself. But the combined strenghts of Rexacor and the Ghostbusters managed to defeat the Stone Golem and instead used the Megalaser to reignite the CoreSun. This in turn caused the Golem to turn back into stones once more. Only the Golem General managed to escape thanks to his pet spider Collywobble. Hiding in a dark cave, the General swore revenge against Rexacor.
CoreWorld first appeared in the episode Inside Out.