For just two days - on June 12 and 13 - after taking either the lift or descending the 455 steps that take them into the Gouffre at Padirac
, visitors will find a pillar box and a fully-functioning post office.
It'll be open for business to coincide with the launch by La Poste, the French postal service, of a limited edition set of 10 stamps promoting the whole region of the Midi-Pyrénées including one with an image of the chasm.
As well as being able to send mail, customers and philatelists will be able to have their collection franked with a special "Padirac Day" or "First Day" seal.
"We've done our research and as far as we know this will be a world first: a post office 75 metres underground," Erik Burté, the director of the post office in the nearby village of Gramat and the man who'll be in charge of the Padirac branch over the two days, told national radio
"I'm in no way an expert at potholing, nor are my colleagues," he continued. "So it'll be a first for us too, to remain underground."
Burté and his colleagues might not have experience working below the surface, but with temperatures at a steady 13 degrees, they'll be wearing the requisite gear for working underground, including a helmet, suitably warm overalls and boots.