Marine One is the
callsign of the
helicopter used by the
President of the United States of America. It can refer to any of 19 helicopters operated by the
HMX-1 "
Nighthawks"
squadron of the
United States Marine Corps—either the roomy
H-3 Sea King or the newer, smaller
H-60 Black Hawk. Over 800 Marine
personnel supervise the operation of the Marine One fleet, which is based at
Quantico, Virginia but is more often seen in action on the
North Lawn of the
White House, or at
Andrews AFB in
Maryland where it is used to connect to
Air Force One for longer journeys.
The first president to get a helicopter was Dwight Eisenhower in 1957, who traveled on a bare bones military rig, an H-13 Sioux, that lacked the air conditioning and lavatory accommodations of its modern equivalent. It was replaced by the H-34 in 1958: the Sea King first flew as Marine One in 1961. Nowadays, Marine One is the preferred alternative to motorcades, which can be expensive and a logistic nightmare compared to the helicopter.
Wherever Marine One flies, it is met on the ground by at least one Marine in full dress uniform, who opens the door in a mechanical fashion and steps aside to let the POTUS by. Bill Clinton flew across the Grand Canyon in Marine One shortly before leaving office, and upon landing in the middle of nowhere, was surprised to find a single Marine covered in brass standing on the edge of the canyon. Having a trained Marine at the door, however, is critical: Gerald Ford bumped his head on the door frame on several occasions.
As an additional security measure, Marine One always travels in pairs: one helicopter carries the president, while the other serves as a decoy for would-be assassins on the ground. To add to the tightness of the Marine One ship, every member of HMX-1 is required to receive a top secret "Yankee White" security clearance before touching any of the helicopters used for presidential travel.
Sources:
http://www.whitehouse.gov/whmo/hmx1.html
http://www.cbsnews.com/stories/2002/09/18/earlyshow/living/travel/main522391.shtml
http://www.globalsecurity.org/military/agency/usmc/hmx-1.htm