Peroni was scanning the piazza for signs of life. Mauro had his camera out, changing films. Costa approved. This was a rare sight, he thought. It deserved to be recorded.

“Where the hell is everyone, Nic?” Peroni asked. “I don’t even see the bums.”

The poor were with you always. Particularly in a place like this.

“Maybe they’re inside already,” Costa replied. Or, even better, perhaps the city had discovered some hidden reserve of compassion and found space to house them for the night.

“We’ve got company,” Peroni said, pointing to a figure emerging from behind the western wall of the building.

The newcomer shivered inside his dark uniform, shielding his face against the snow, which seemed to have found newly energized vigour. The caretaker stumbled forward, stared at them hopefully, then asked, “You the cops?”

Peroni waved his badge. Costa looked around the square again. More people should have been there. Falcone ought to arrive soon, too.

“I’m not going inside on my own,” the caretaker said. “Some of these scum use knives.”

Peroni nodded at the doors. “Best open them up, then.”

The man let loose a dry laugh, then looked at Sandri, once again aiming his camera right, left and centre. “Sure, Officer. That’s all it takes. Is your man here going to shoot some pictures too? They say you see it just once in a lifetime. Snow coming down like that, straight through the eye.”

“So what are we waiting for?” Peroni asked.

Costa knew the problem. Behind the portico lay the largest pair of imperial Roman doors in existence. Worked bronze, almost as high as the porch itself, and more than a metre deep. Sometimes, before going on duty, he’d take a coffee in the square in the early morning, watching the Pantheon being prepared for another day of crowds. No one who worked in the building ever approached through the front, not to begin with. The doors opened inwards, their mass being drawn back slowly from behind.

“We need the tradesmen’s entrance,” Costa said.

“Precisely.” The caretaker sniffed, then drew back his collar to reveal a gnarled, florid face that looked as if half a bottle of grappa could be wrung out of it. “All three of you coming?”

Costa looked at Peroni. “I can handle a couple of street people. You stay here with Mauro. Wait for Falcone.”

“No,” Peroni said, striding out of the snow and towards the shelter of the portico. “I stay here.”

Costa followed in the caretaker’s swift footsteps, walking to the western flank, where they descended some stairs down to what must have been the original level of the city when the Pantheon was built. There was a locked iron gate, then further steps and a long, narrow path, in the shadow of the high modern wall of the adjoining street, to a small, secure door almost at the rear of the building.

“The tradesmen’s entrance,” the caretaker announced icily, then turned a couple of locks and threw it open. Costa stepped into the alcove and waited as the man fumbled with some keys at a second door, which led, he guessed, to the great circular interior. He wondered briefly what kind of bum locked the doors behind him.

He listened to the metal tumblers turn.

“After you,” the caretaker said. “I’ll get the lights.”

Nic Costa walked into the darkness and felt the chill of fresh winter air on his face. The night breeze was circling in the vast hemisphere he knew lay before him. And there was another sound too. Of a human being moving: short, anxious steps in the blackness beyond.

He felt his jacket, wondered about the gun. Then the lights of the building burst into life, bringing a sudden harsh sun into the shadows of the vast, airy, artificial universe enclosed beneath the ancient structure’s huge dome.

Someone cried out with surprise. A young voice. The noise reverberated around the vast emptiness so quickly it seemed to come from everywhere.

“Will you look at that?” the caretaker said, no longer thinking about the intruders.

Through the giant open eye of the oculus of the roof came a steady, swirling stream of snow, pirouetting around itself with the perfect, precise symmetry of a strand of human DNA.

It fell in the dead centre of the room, where an inverted, icy funnel was growing, spreading out beyond the central marble ring and rising, at its peak, to a metre or more.

Costa heard movement to his right. A slight, small figure dashed through a brilliant yellow beam cast down by a spotlight near the main altar, then fled into the pool of shadows in a recess on the far side of the building.

“Scum,” the caretaker muttered. “What are you going to do?”

Costa had been running the options through his mind. Chase some lone, cold, hungry bum through the darkness of Hadrian’s holiest of holies? And all for what?

“Open the doors,” he said. “The main ones.”

Costa was walking towards them already, anxious to enjoy the look of astonishment on the faces of Gianni Peroni and Mauro Sandri when those gigantic bronze shutters were pulled back to reveal this wonder to the world on the other side.

“What?” the caretaker asked, putting a hand on Costa’s shoulder until something in the detective’s eyes told him this was not a good idea.

“You heard!” Costa snapped, getting angry with the man, wondering what he thought he was protecting here.

There were more keys and some kind of electronic monitor needed attention. Costa got on his cell phone and called his partner, just beyond the doors.

“I think it’s just a kid, Gianni,” he told Peroni. “If he runs, you can get some exercise. Otherwise… hell, it’s almost Christmas.”

The big man’s laugh came back as a double echo, from the phone and, fainter, from beyond the doors. “You’re itching to do Leo’s clean-up statistics some good tonight.”

“Just stand back and watch when we open this place.” Then Costa thought about what Peroni had said. “Is Falcone there?”

“Walking right across the square. And on the phone too. This isn’t a conference call or something?”

Costa heard a low metallic groan and slid the phone back in his jacket. The caretaker was heaving at the bronze behemoth on his right, tugging it back on a set of ancient hinges. Costa took hold of the second door and pulled hard at the handle. It moved surprisingly easily.

In the space of a few seconds they had the doors open. The night wind slammed straight up the portico and flung snow into their faces. Nic Costa brushed the stinging flakes out of his eyes. Gianni Peroni stood there, clearly transfixed by what he saw. Sandri was a few steps behind him, tense, upright, firing off photos constantly. Falcone had arrived too and seemed to be barking angrily down the phone.

Costa turned round and took another look at the magical scene behind him, snow swirling down from the heavens, as if tethered to some magnetic, twisting beam of light.

The vagrant was moving in the Pantheon now. Nic Costa no longer cared. He stood back from the door to let the intruder run, to break out from this tight, enclosed universe that was the dream of an emperor who had been dead for nearly two millennia.

Then he looked outside again and recognized a different shape-upright and stiff on the steps of the fountain-not quite able to believe what he was seeing, to reconcile it with this bewitching night.

A figure slipped past him, brushing against his jacket. Costa didn’t even look. With fumbling fingers he unzipped his coat, felt for his gun in the holster.

“Get down,” he said, still trying to marshal his thoughts, letting the words slip from his mouth so softly he doubted the caretaker even heard. Then he took a deep breath and yelled, as loudly as he could, “Gianni! Get down for Christ’s sake!”

Instinctively, without planning the move, he dashed out into the portico and felt the freezing wind bite at his face. Gianni Peroni was still staring into the interior of the Pantheon, ugly face alight with joy, grinning like a kid. Falcone was getting close to him too, his stern, immobile features for once rapt, enthralled by the scene inside.