Journalists were offered the rare glimpse of the 24 metre diametre steel tubes at the National Centre for Aeropsace Research (Onera), where propellers, weighing over a tonne each, power the huge gusts of wind.
Despite their size the fans can get up to speeds of 230 revolutions every minute – nearly four a second – at the world’s largest wind tunnel that tests aircraft to their limit.
The simulations last for just a matter of seconds, but the four tunnels in Modane allow manufactures and defence ministries to test hypersonic flight, sometimes exceeding 12 times the speed of sound.
The huge site was built out of sight with extreme precision after it was discovered during World War Two.
Jean-Paul Becla, test director at the site in Modane, eastern France, said: ‘A French mission in 1945 discovered a tunnel being built in Austria by the Germans. It chose to take it apart and put it in France, aware that it was a major issue.
‘The site, chosen for its air quality, also has the advantage of being close to dams producing enough energy for the wind tunnels, which alone consume one thousandth of France’s electric capacity.