As we can understand from the passage, it was assumed many centuries ago that the large land mass around the North Pole ____

In earlier centuries it was thought that a great continent
must exist in the southern hemisphere, around the South
Pole, to balance the known land masses in the north. Its
real extent was better understood in the 18th century,
particularly when Captain Cook sailed for the first time
south of the Antarctic Circle and reached the edge of the
icepack. A portion of the ice-covered continent was first
sighted by Edward Bransfield in 1820. Explorers of several
other nations also sighted portions of the coast-line in
other quarters and wrote detailed accounts of their
observations. However, in the light of these accounts, the
first extensive exploration was made by Captain James
Clarke Ross in 1841 when a great part of the Antarctic was
discovered.