The Clear Lake City STOLport is no longer in existence. The Walt Disney World resort in Florida was also served with scheduled airline flights operated with Twin Otter aircraft. The Walt Disney World Airport, also known as the Lake Buena Vista STOLport, was a private airfield constructed by The Walt Disney Company with Twin Otter operations in mind. In the early 1970s, Shawnee Airlines operated scheduled Twin Otter flights between the Disney resort and nearby Orlando Jetport, now Orlando International Airport, as well as to Tampa International Airport. This service by Shawnee Airlines is mentioned in the "Air Commuter Section" of the 6 September 1972 Eastern Air Lines system timetable as a connecting service to and from Eastern flights. This STOL airfield is no longer in use. Another commuter airline in the United States, Rocky Mountain Airways, operated Twin Otters from the Lake County Airport in Leadville, Colorado. At an elevation of above mean sea level, this airport is the highest airfield in the United States ever to have received scheduled passenger airline service, thus demonstrating the wide-ranging flight capabilities of the Twin Otter. Rocky Mountain Airways went on to become the worldwide launch customer for the larger, four-engine de Havilland Canada Dash 7 STOL turboprop, but continued to operate the Twin Otter, as well. Larger scheduled passenger airlines based in the United States, Canada, Mexico, the Caribbean and Australia, particularly jetliner operators, also flew Twin Otters, with the aircraft providing connecting feeder service for these airlines. Jet aircraft operators which also flew the Twin Otter included Aeronaves de Mexico, Air BC, Alaska Airlines, ALM Antillean Airlines, Ansett Airlines, Cayman Airways, Frontier Airlines, LIAT, Norcanair, Nordair, Ozark Air Lines, Pacific Western Airlines, Quebecair, South Pacific Island Airways, Time Air, Transair, Trans Australia Airlines (TAA), Wardair and Wien Air Alaska. In many cases, the excellent operating economics of the Twin Otter allowed airlines large and small to provide scheduled passenger flights to communities that most likely would otherwise never have received air service. Twin Otters are also a staple of Antarctic transportation. Four Twin Otters are employed by the British Antarctic Survey on research and supply flights, and several are employed by the United States Antarctic Program via contract with Kenn Borek Air. On 24–25 April 2001, two Twin Otters performed the first winter flight to Amundsen–Scott South Pole Station to perform a medical evacuation. On 21–22 June 2016, Kenn Borek