Declaring its 160-year mission of Antarctic exploration accomplished, the U.S. Navy lowered its flag Friday and shut down "Operation Deep Freeze."

The Navy has been winding down its Antarctic operations for years, transferring its duties to the next tenant of the icy continent: the U.S. Air National Guard.The transition was completed Friday in a ceremony in Christchurch, the operation's headquarters since 1955. During the event, Navy officers handed the official flag of the U.S. Antarctic Program to officers from the Air National Guard, responsible for U.S. arctic operations at the other end of the globe since 1975.

Navy officers also laid a wreath commemorating the 50 Americans who have died in Antarctica since the Navy renewed its modern expeditions in 1955. As a permanent monument to the dead, the officials unveiled a brass plaque in the shape of the Antarctica, with the names of those who died.

The fledgling Navy sent its first expedition south in 1838, when Lt. Charles Wilkes left to explore about 1,200 miles of the Antarctic Peninsula.

The United States then lost interest in Antarctica until the 20th century, when Navy aviator Richard E. Byrd, who claimed to have flown over the North Pole in 1926, went the other direction and reported an overflight at the South Pole in 1929.

In 1946, Byrd led the massive Navy "Operation Highjump," an exercise to prepare the U.S. military to fight the Soviet Union in polar conditions. Antarctica was chosen for the mission because war games in Alaska or Canada would have been too provocative.

During the operation, Byrd invaded Antarctica with 4,700 men on 13 ships and 33 aircraft, pioneering the use of helicopters and icebreakers. It is still by far the biggest expedition ever sent to the continent.

Byrd's activity in Antarctica from 1928 to 1957 made the United States the most active nation there. Today, about half of the continent's entire summertime population of about 3,000 are Americans.

The Navy's support force for the continent was popularly known as "Operation Deep Freeze," and its rugged members included the "Ice Pirates," six Bell Huey helicopters that have airlifted scientists and their cargo to remote stations for years.

Priding themselves as being elite pilots in the world's most challenging flight conditions, the Ice Pirates survived shifting winds, downdrafts and whiteout conditions.

Other pilots known for their bravery and skill belonged to the Navy's VXE-6 flight detachment, which flew six aging C-130 prop-jet Hercules transport planes. The workhorses of Antarctica, the planes brought in supplies and people, landing on either skis or wheels, depending on whether they set down on snow or an ice runway.