Shihab smiled. ‘Ah, Hakim, it is impossible to keep the dust out at this level. But just think – those very particles could be all that remains of a former king or prince of Persia.’

Hakim blew his nose – Persian king or not – on a dirty brown handkerchief. He had just pushed it back into his pocket when they came to a steel miner’s cage floored with thick rubberised matting. They entered the cage and Shihab pushed the single smooth lever to the downward position. The cage dropped silently into the darkness. Shihab smiled with pride, counting the passing layers of toughened concrete and lead shielding. The facility was all his own design, built to give off as small an energy signature as was possible. He knew full well the capabilities of the US spy satellites – high-resolution digital images were only one of their talents. These days they could sniff out heat, power and radiation signatures to fifty feet below the surface. So the Iranians needed to be careful, needed to go deeper.

After many minutes, the cage slowed with a hiss and stopped at a darkened corridor and a fortified steel door, considerably stronger than the one they had come through at the surface. Shihab entered another code and hidden rollers pulled the large metal slab out of the way. He closed his eyes briefly as a blast of negative air pressure rushed past him, only opening them when he perceived strong light on his eyelids. Before them was a chamber of almost surgical whiteness.

Shihab and Hakim knew what was expected, having performed this ritual many times. They both sat on the benches provided and removed their shoes. From a recessed cupboard they drew out particle-free garments, donned lightweight polymer all-over suits, pulled rubber-soled shoes over their feet, and used a small towel lightly moistened with demineralised water to wipe their faces, neck and hands.

When Hakim had finished, Shihab clipped a small radiation badge to the soldier’s breast pocket. ‘They’re the latest – each one contains a small sheet of radiation-sensitive aluminium oxide. When it’s exposed to radiation, the tag shines with a visible blue glow. We all have to wear them now. Let’s hope they don’t shine for us today, yes?’

The two men moved to the facility’s final access stage: a glass booth with another metal door beyond. The entry requirements here were far more stringent – as well as keying in a numeric code, Shihab had to lick his thumb and place it over a small mesh circle on the entry pad. Fingerprints and DNA were obtained, scanned and verified before the curved glass door slid back. Anyone trying to access the facility without proper authorisation would find himself locked in the chamber as it rapidly filled with lethal tabun gas. Clear and odourless, the gas immediately shut down the human nervous system, and then just as quickly dissipated, allowing safe removal of any bodies.