There was nobody behind the door, and nothing except a downward-sloping stairway shaft with a long flight of concrete steps leading steeply underground. The walls were smooth and white, and the stairway was lit by small LED lights embedded in the ceiling. The space-age effect was a striking and surreal contrast to the crumbling ruins above ground.

At the bottom of the steps the shaft curved away to the right, inviting them to follow. Ben tentatively led the way down, Wolf following. Behind them the steel door glided shut and locked with a click. There was no handle on the inside, either. Like the door of a cell. No going back now, even if Ben and Wolf had wanted to.

Ben reached the foot of the steps and estimated that they were about ten metres underground. He peered around the bend and saw that the shaft continued sloping gently deeper, leading to a second steel door that barred the way. Unable to shake off that strange feeling of being watched, he looked around him. The smooth white ceiling was maybe seven feet high, and just below it were parallel rows of steel mesh grids that he at first assumed were just ventilator shafts, until he spotted the eye of a camera glinting from behind one of them.

The second steel door unlocked and glided open as they approached. On the other side of it was a square white chamber that made Ben think of an airlock in a biochemical lab. The whoosh of air conditioning caressed his skin. More cameras watched them from above. Panels were set into the walls on each side and a third door stood shut in front of them. Before the exit was a steel archway that Ben guessed was a higher-tech version of the metal detectors used in airports. As Wolf stepped into the chamber, the second door started closing behind them. It was a claustrophobic feeling, like being trapped in a stalled lift.

Ben almost had to pinch himself to be convinced he was seeing this for real. Whatever the hell this facility was, it must have cost millions to create. He’d been in secret military installations that were less sophisticated. And he was getting the feeling that there was a lot more to come.

Wolf was having the same thoughts. ‘This place is unbelievable,’ he muttered.

‘I’m glad you like it,’ said the voice of Vincent Eritas, coming deep and resonant from a speaker behind one of the wall grilles. Ben imagined their mysterious host sitting in some hidden control room, watching them on a monitor as he talked into his mic.