In 1768, Lieutenant James Cook set sail on the HMS Endeavour, officially tasked with a crucial scientific mission: observing the rare Transit of Venus from Tahiti to help measure the size of the solar system. But packed away in Cook's cabin were sealed, secret orders from the Admiralty. Once the astronomical observations were complete, his real journey began—the hunt for the legendary, uncharted Terra Australis Incognita, the "Great Southern Continent." In this episode, we trace this pivotal voyage, moving from celestial mechanics in Polynesia to the groundbreaking mapping of New Zealand and the eastern coast of Australia.