A fairly recent issue of Airliners magazine stated when asked the same question by a reader that it was Upington, South Africa - details below from this hot link
The runway is 16 076 feet long! Do note take too much note of all the disclaimers. The author of the site is clearly just covering himself! I have just copied it over into the forum and the whole layout has re-arranged itself and the columns have fallen away, but you can get the picture; alternatively, visit the site.
Worldwide Airport Path Finder
This is a database tools demonstration by Falling Rain Genomics, Inc. The information presented here may be inaccurate or obsolete, and is not valid for navigation or flight planning. No warranty of fitness or merchantability for any purpose is made or implied. FAUP
IATA code: UTN South Africa
Latitude: 28° 24' 3.449S (-28.400958) Longitude: 21° 15' 38.948E (21.260819)
Type: Civilian Elevation: 2791 feet MSL
Mag Var: 18.675 W
MET on file for FAUP
1 hours 21 minutes ago FAUP winds calm vis good, at least 6sm temp 32C dewpoint 13C
TAF on file for FAUP Time:1421Z
FAUP 300900Z 301812 04005KT 9999 FEW050CB
BECMG 1820 CAVOK
Com Name Freq Sector Hours Remarks
APP UPINGTON APP CON 121.3M 0430-1800Z MON-WED; 0330-1800Z THU-FRI, 0830-1030Z AND 1400-1800Z SUN.
MISC UPINGTON APRON 122.65M H24
TWR UPINGTON TWR 121.3M 0430-1600Z MON-WED; 0330-1800Z THU-FRI; 0830-1030Z AND 1400-1800Z SUN.
RWY 19/01 8000x150 ASP
RWY 35/17 16076x197 ASP
RWY 26/08 6000x150 COM The information for this airport was pieced together from unverified internet sources of uncertain quality and applicability. The airport may be significantly misplaced, or misidentified, or there may be no airport at all.
No fuel information.
Weather Underground for Upington (0 nm S)