Many of the shafts had been filled in completely over the years. But some of them hadn’t. These workings were centuries old, and a few had been lost and forgotten – lost, that was, until someone stumbled on a loose stone and broke a leg, or slipped through a corroded capping plate and disappeared into the ground for ever.

Cooper put the Toyota back into gear and drove on over the moor, heading inexorably towards the clouds of black smoke on the skyline.

On Oxlow Moor, some of the firefighters were dousing smouldering hotspots with water from backpacks like garden sprayers. Others were stamping and kicking out the smaller fires, or flailing them with beaters.

Cooper found the incident commander by his white helmet and white tabard. He turned out to be the watch manager from Edendale fire station. This was a major incident, so somewhere there would be a Level Two commander in overall charge.

‘How dense is the smoke up there?’ asked Cooper.

‘How dense? You can’t see your hand in front of your face,’ said the fire chief, pushing back the visor of his helmet.

‘What’s the current Fire Severity Index?’

‘The FSI has been at five for the past two days. It can’t get any higher.’

Several square miles of moorland were burning now, with dense smoke trailing across the sky. At least barriers were out at strategic points along the adjacent roads to stop traffic. Some areas where earlier fires had started had been dampened down after a huge operation, but the ground was still smouldering.

Cooper could see a silver-grey ranger’s Land Rover Defender towing a water bowser on to the moor, and one of the national park’s eight-wheel-drive Argo Centaurs operating alongside the fire service’s Unimog all-terrain tender.

Derbyshire Fire and Rescue Service were only part of the operation when it came to a moorland fire like this one. National park rangers, National Trust wardens, water companies and other major landowners all became involved. They’d come together to form the Fire Operations Group more than fifteen years ago, after a serious moorland blaze. They drew up joint plans, shared specialist equipment and worked side by side to tackle major fires.

The fire chief shook his head at the scene on the moor. ‘I’d be a lot happier if you could get hold of the people who caused these fires.’

‘They aren’t accidental?’

He laughed. ‘Accidental? I’m not convinced you could even do this accidentally. It isn’t so easy to start a moorland fire just by dropping a match or something. When you drop a spent match or a cigarette end, it’s almost always on a path anyway. Bare earth or rock. Nothing that burns easily. These fires began way out in the middle of the dry heather, where they had the best chance of catching. If we’d found the remains of any Chinese lanterns, I might accept it as accidental.’

‘Chinese lanterns? Really?’

‘Absolutely. There’s been a complete craze for them recently. It’s mad. I mean, what is a Chinese lantern? You’re basically lighting a candle inside a paper bag and letting it drift off wherever the wind takes it. People send off whole swarms of them at once. Then they land on someone’s crop, or on a baking-dry moor like this, and the result is no surprise to anyone. Certainly not to me. And yet they call that an accident. Well, not in my book – it’s sheer recklessness with someone else’s property. They’re talking about banning the things in some places, and it’s none too soon in my opinion. It’s already the case in other countries, even in China.’

Cooper remembered his brother complaining about Chinese lanterns too. Of course, Matt complained about a lot of things. But the National Farmers’ Union had said the lanterns were not only a fire hazard, but could also wreck farm machinery, or be chopped up and get into animal feed, with potentially fatal results for livestock.

‘But no signs of Chinese lanterns in this case? No one been holding a party and letting them off?’

‘Not so far as we can see,’ said the fireman. ‘There’d be wire frames left, even after they’d burned up.’

‘Arson, then?’

The watch manager shrugged. ‘Without a confession, there’s no way anyone can actually prove the fires were started deliberately.’

‘But that’s your gut instinct?’

‘Yes. But my gut instinct isn’t proof of anything.’

‘Fair enough.’

‘Our own fire investigator is on his way, but we’ve narrowed down the location where we think the fire started. Or was started. Whichever. We believe there are traces of accelerant use.’

‘Petrol? Lighter fluid?’

‘Something of that nature. The burn pattern is distinctive. A higher rate of combustion, a greater degree of heat. In that one patch, the fire has just left ashes.’

‘Can we have a look?’

‘Sure. Just take care.’

As they walked, the fireman pointed up the slope, where the heather and bracken had been burnt off completely, leaving a blackened stretch of ground devoid of vegetation of any kind.

‘Next thing, we’re going to have the archaeologists poking about up here,’ he said.

‘Why?’

‘Remains of some old stone buildings are showing through where the fire has caused most damage to the ground cover.’

‘Really?’

Cooper took a few steps up the slope to see more closely. Bare peat was visible in many places, and he could just see a line of muddy masonry protruding from the eroded surface. From this angle, it did look like the remains of a wall, or the foundations of a vanished structure.

‘How old?’ he asked.

‘No idea, Sergeant. But I’m sure there’ll be no shortage of people wanting to come out here and tell us. It could just be some old shepherd’s hut. On the other hand …’

‘Yes, it could be anything. There’s supposed to be an abandoned medieval village around here somewhere. There are always Roman sites turning up. We’re only a stone’s throw from Batham Gate, the old Roman road. There could have been a small fort here, for all we know. It’s an ideal position. Look at the vantage point they would have had.’

The fireman shrugged. ‘I wouldn’t know. I just hope we don’t start getting the blame for any damage that’s been done to it.’

Cooper knew that Oxlow Moor had a lot of history. Some of it, though, was more recent, and less harmless than a few passing Roman legionaries.

He turned and looked across the moor. The Light House wasn’t visible from here, because of the shape of the land. It must be over a mile away from his position.

‘I passed the old pub on the way here,’ he said. ‘There was a report of a break-in.’

‘That’s right. We used the place as a rendezvous point earlier, but the fires have been moving this way pretty fast, as you can see. The prevailing wind is moving to the east.’

There was no mistaking the path of the fire. A huge tract of charred heather and bracken had been left in its wake as the flames advanced across a wide front. It looked as though an invading army had passed through, leaving nothing but scorched earth behind them.

‘Well I can check the pub on my way back,’ said Cooper. ‘I pass fairly close to it.’

There was a disturbance among the firefighters and rangers further up the hill. Someone called down and waved a hand in an urgent gesture.

‘What’s going on now?’ asked Cooper.

‘Oh Lord. It looks like they’ve found something else.’

‘More archaeological remains?’

‘Chief,’ shouted one of the firemen, ‘you might want to take a look at this.’

Out of curiosity, Cooper followed the watch manager up the hill through the remains of the burnt heather to where the firefighters had gathered. And within minutes he’d forgotten all about the break-in at the Light House.

A couple of hours later, the scene of the find on Oxlow Moor had been taped off, but only by driving plastic stakes into the burned peat around it. The taping seemed a bit unnecessary in view of the nature of the surroundings, but at least procedure was being followed. E Division’s crime-scene manager Wayne Abbott was present, which indicated the seriousness with which someone had responded to the finds. Cooper had been joined by Carol Villiers, dispatched from West Street on his call.