SYDNEY, Australia — It’s about half the size of the United States, and it’s been hiding under everyone’s noses — or more precisely, under the waves — for millions of years. Now, scientists are setting sail to finally help solve the mystery of Zealandia, the lost undersea landmass being billed as the world’s eighth continent.

Zealandia, an expanse of 1.9 million square miles, extends from far south and east of New Zealand up to New Caledonia and west to an area off Australia’s northeast coast. It was part of Australia until about 75 million years ago, when it started to break away and move northeast. That movement stopped 53 million years ago, and scientists have slowly discovered the landmass, almost entirely submerged, over the past two decades.

“It’s a long way from anywhere,” said Rupert Sutherland, a Victoria University of Wellington professor who will be on the monthslong voyage from Australia to Zealandia, which began Friday. “A few missions have been going there to look for some specific things, but there hasn’t really been a coordinated plan of attack.”

He continued, “It is quite exciting, this Zealandia exploration. We’ve got an entire continent that has not been explored.”