For two days, oxygen-starved Russian troops crawled nine miles through a pitch-black underground pipeline, creeping behind enemy lines and springing a surprise attack that wiped out the enemy in a blaze of bullets.

The Ukrainian soldiers had no idea that Operation Potok was coming.

Hundreds of men streamed out of the pipeline, storming Ukrainian positions north of Sudzha, pushing them back out of Russia in a triumphant counter-offensive to save the motherland.

It was Russia’s elite forces who “burst out unexpectedly, like demons – black, dirty, and exhausted”, according to a soldier with the call sign “Mowgli”.

The “soldiers from the pipe” became the stuff of legend, and those who returned were welcomed home as war heroes.

“Blow up all your pipes out of fear... we’ll still come to you from under the ground,” goes a new song about the mission, first performed outside a church in central Russia, where a 50ft replica had been installed for people to admire.