‘We’ll follow them and see. But first, let’s find out what they were up to here.’ He pulled up and pushed his door open, then pointed at the tracks. ‘Two men, see? Been back and forth a few times – fetching something, or leaving it here. So let’s have a squiz. There’s a torch in the glove box – can you grab it?’

‘Of course.’ She handed it to him and they stood for a moment inspecting the hillside. The opening of the closest cave was quite small – she’d need to stoop to enter it, Tilly thought, and Connor would be bent double. The entrance looked black in contrast to the yellowed rock. She felt a momentary reluctance at forsaking the bright day for the gloom within, but Connor was heading for it and she followed him willy-nilly. She’d wanted to be here, she told herself, so it was silly to be intimidated by mere darkness.

The first thing that struck her was the surprising size of the hollowed area once they were within it, for the roof rose and the sides widened almost immediately. There was an earthy smell and the air, surprisingly, was oppressively warm and heavy with humidity. ‘Whew! It’s stifling!’ she exclaimed. ‘I thought caves were cold.’

Connor was playing the torchlight over the walls, the moving beam glistening on the rock. ‘It’s wet.’ He touched it, then peered closer. ‘Must be constant seepage. It accounts for the humidity – that and the small entrance, I suppose. There’s moss and liverworts growing, and some sort of fungus that gives off light.’ Without warning he switched the torch off, plunging them into darkness. Tilly gasped but then she could see it too, a faint, barely discernable bluish glow, almost not there in the stygian blackness. She felt the sweat beading on her face and gasped in the soggy air.

Abruptly the beam of light returned. ‘Boy, sure is muggy.’ Connor was moving away, deeper into the cave as he spoke, his footsteps muffled. ‘The mass of the hill must hold the heat in. The stone soaks it up and releases it inwards, I guess.’

‘How far back does it go?’ Tilly hurried after him, resisting the urge to grab his arm. She liked nothing about this place – not the darkness, nor the booming echo that followed her question, or the sudden flurry of leathery wings above her head. She swallowed a cry, managing only to squeak, ‘Bats!’ They were harmless, she told herself firmly, as Connor flashed the light up on the whirring cloud of tiny creatures they had disturbed from their roosts on the rocky ledges and roof of the cave. ‘Luke would love this,’ she said, determined to act like a proper ranger. ‘We’ve got flying foxes, of course – thousands of them – and we see ghost bats sometimes when the moon’s full, but I’ve never noticed little ones before.’