If so, it was increasingly obvious to Tom that this was an argument that nature was winning, decades of neglect having left monuments eroded by pollution and tombs cracked open by weeds and the cruel ebb and flow of the seasons. In one place, a pine tree had shed a branch, the diseased limb collapsing on to a grave and smashing its delicately engraved headstone into pieces. In another, the ground had risen up, snapping the spine of the vault that had dared to surmount it. And now it seemed to have swallowed Allegra’s signal too.

‘Where was the last reading from?’ Dominique asked, ever practical.

‘Over there-’ Tom immediately broke into a loping run, vaulting the smaller graves and navigating his way around the larger tombs. Then, just as he was about to emerge into one of the wide avenues that cut across the cemetery, he felt Archie’s hand grab his shoulder and force him to the ground.

‘Get down,’ he hissed.

Three men had emerged from the trees ahead of them, their machine guns glinting black in the moonlight, torch beams slicing the darkness. Moving quickly, they glided over to a large family vault, their boots lost in the long grass so that they almost appeared to be floating over the ground. As Tom watched, they ghosted up its steps and vanished inside.

‘She must be in there,’ Tom guessed, standing up.

The vault was a small rectangular building designed to echo a Roman temple, a few shallow steps leading up to the entrance, a Doric frieze carved under the portico, white Travertine walls decorated with columns that gave the illusion of supporting the tiled roof. The entrance was secured by a handsome bronze door that the elements had varnished a mottled green. A single name had been carved over it: Merisi. Tom pointed at it with a smile as they crept towards it.

‘What?’ Dominique whispered.

‘Merisi was Caravaggio’s real name.’

They paused, straining to hear a voice or a sound from inside. But nothing came apart from the silent echo of darkness.

With a determined nod at the others, Tom carefully eased the door open with one hand, his gun in the other. This and three other ‘clean’ weapons had been sourced by Archie from Johnny Li while they had been watching the hangar at Rome airport. The price had been steep-the money he claimed Tom still owed him, plus another ten for his trouble. Archie had only just stopped cursing about it, although Johnny had at least held his half of their earlier bargain and returned Tom’s watch.

Inside, a thin carpet of dirt and leaves covered the black-and-white mosaic floor and lay pooled in the room’s dark corners. At the far end stood a black marble altar with the name Merisi again picked out in bronze letters above a date-1696. In front of this were two high-backed prayer stools, once painted black and upholstered in a rich velvet, but now peeling and rotted by the cold and the damp. Above the altar, suspended from the wall, was a crucifix, one arm of which had broken off so that it hung at an odd angle.

The room was empty.

‘Where the hell have they gone?’ Archie exclaimed, rapping the walls to make sure they were solid.

Tom examined the floor with a frown.

‘How did they expect to bury anyone in here?’

‘What do you mean?’ Dominique frowned.

‘It’s a family vault. There should be a slab or something that can be lifted up.’

‘No inscriptions either,’ Archie chimed in. ‘Not even a full set of dates.’

‘And the one that’s here doesn’t fit,’ Dominique pointed out. ‘This graveyard wasn’t used until the 1730s. No one would have been buried here in 1696.’

‘It could be a birth year,’ Tom suggested, crouching down in front of the altar. ‘Maybe the second date has come away and…’

The words caught in his throat. As he’d rubbed the marble, his fingers had brushed against the final number, causing it to move slightly. He glanced up at the others to check that they had seen this too, then reached forward to turn it, the number spinning clockwise and then clicking into place once it was upside down so that it now read as a nine.

Archie frowned. ‘1699? That doesn’t make no sense either?’

‘Not 1699-1969,’ Tom guessed, turning each of the previous three numbers so that they also clicked into place upside down. ‘The year the Caravaggio was stolen.’

There was the dull thud of what sounded like a restraining bolt being drawn back from somewhere in front of them. Then, with the suppressed hiss of a hydraulic ram, the massive altar began to lift up and out, pivoting high above their heads, stopping a few inches below the coffered ceiling.

They jumped back, swapping a surprised look. Ahead of them, a flight of steps disappeared into the ground.

20th March-10.37 p.m.

The steps led down to a brick-lined corridor set on a shallow incline. It was dimly lit, the sodium lighting suspended from the vaulted ceiling at irregular intervals forming pallid pools of orange light that barely penetrated the cloying darkness. In places the water had forced its way in, the ceiling flowering with calcite rings that dripped on to the glistening concrete floor.

Treading carefully, their guns aiming towards the darkness into which the three armed men who had preceded them down here had presumably disappeared, they crept down the tunnel. Tom had the vague sense that they were following the contour of the Aventine as it rose steeply to their right, although it was hard to be sure, the passage tracing a bewildering course as it zigzagged violently between the graveyard’s scattered crypts and burial chambers. Eventually, after about two hundred yards, it ended, opening up into a subterranean network of interlinking rooms supported by steel props.

‘It’s Roman,’ Dominique whispered, stooping to look at a small section of the frescoed wall which hadn’t crumbled away. ‘Probably a private villa. Someone rich, because this looks like it might have been part of a bath complex.’ She pointed at a small section of the tessellated floor which had given way, revealing a four-foot cavity underneath, supported by columns of terracotta tiles. ‘They used to circulate hot air through the hypercaust to heat the floors and walls of the caldarium,’ she explained.

They tiptoed through into the next room, their path now lit by spotlights strung along a black flex and angled up at the ceiling, the amber glow suffusing the stone walls. Dominique identified this as the balneum, a semicircular sunken bath dominating the space.

Picking their way through the thicket of metal supports propping the roof up, they arrived at the main part of the buried villa, the tiled floor giving way to intricate mosaics featuring animals, plants, laurel-crowned gods and a dizzying array of boldly coloured geometric patterns. Here, some restoration work appeared to have been done: the delicate frescoes of robed Roman figures and carefully rendered animals showed signs of having been pieced back together from surviving fragments, the missing sections filled in and then plastered white so that the fissures between the pieces resembled cracks in the varnish on an old painting.

An angry shout echoed towards them through the empty rooms.

‘You think Santos is already here?’ Dominique whispered.

‘Allegra first,’ Tom insisted. ‘We worry about Santos and the painting when she’s safe.’

They tiptoed carefully to the doorway of a small vaulted chamber. The walls here had been painted to mimic blood-red and ochre marble panels, while the ceiling had been covered in geometric shapes filled with delicately rendered birds and mischievous-looking satyrs. And crouching on the floor with their backs to them, checking their weapons and speaking in low, urgent voices, were the three men they’d seen earlier.

Tom locked eyes with Archie and Dominique; both of them nodded back. On a silent count of three, they leapt inside and caught the three men completely cold.