The Space Colony ARK, also referred to as The ARK, is a space station.
Under Eggman's Control
At some point in the past, Dr. Eggman used the Space Colony ARK to blow up half of the moon by firing its Eclipse Cannon in order to show his power. Eventually, Eggman teamed up with Sonic the Hedgehog, Miles "Tails" Prower, Knuckles the Echidna, Rouge the Bat, and Shadow the Hedgehog to save the world. (StH: #5)
After meeting Dr. Eggman in Windmill Village, who had lost his memories and went by the name of Mr. Tinker, Sonic the Hedgehog mentioned the ARK along with other things while questioning him over his forgotten memories. (StH: #5, #6)