Terra incognita

From EU3 Wiki

Terra incognita (TI) means "unknown territory" and refers to the part of the world not known to the player, represented in-game by a white blank. Explorers and conquistadors are the only ones who can travel into TI, although given time maps of TI provinces will spread into countries that do not yet know them. Every discovery is shown to all the members of the same tech group after 50 years.

Some provinces can never be discovered and are known as permanent terra incognita (PTI). Good examples of places you can't reach: everything inside Africa, tropical jungles in South America, Himalayas and such.

To explore Terra Incognita you need explorers and conquistadors. You employ them just like generals and admirals. They are available after you have Quest for the New World National Idea (early choice of Portugal and Spain). Explorers lead fleets through unknown sea areas, conquistadors do the same on land with their armies. Exploring TI, be it land or sea, improves your naval tradition and adds points to your prestige.