Terra incognita refer to the hypothesized continent Terra Australis Incognita ("The unknown land of the South"), as seen in the Theatrum Orbis Terrarum map by Abraham Ortelius (1570). During the 19th century "terra incognita" disappeared from maps of the earth, since both the coastlines and the inner parts of the continents had been fully explored. But the bottoms of the seas remain mostly un-mapped, as do many other land surfaces in the solar system. The phrase is now also used metaphorically by various researchers to describe any unexplored subject or field of research. For more history of the terms, see Terra Australis,
Terra Mariana Terra Mariana Terra Mariana (Medieval Latin for "Land of Mary") was the official name for Medieval Livonia or Old Livonia (, , ), which was formed in the aftermath of the Livonian Crusade in the territories comprising present day Estonia and Latvia. It was established on 2 February 1207, as a principality of the Holy Roman Empire but lost this status in 1215 when proclaimed by Pope Innocent III as directly subject to the Holy See. Terra Mariana was divided into feudal principalities by Papal Legate William of Modena: After the 1236 Battle of Saule the surviving members of the