Crawling after, Valens noticed that Aulus’s knife was again in his hand. In this enclosed tunnel, nothing could get at Valens past Aulus. He left his dagger in its sheath.

The tunnel was long and it was not straight. After a time it divided into two. Aulus looked back for instructions. There was nothing to choose between the passages. Valens shrugged, then pointed to the left. They crawled on over the smooth rock. Eventually the enclosing walls fell back.

Very little light filtered into the next chamber, but they could sense its echoing size. From somewhere in the darkness came the sound of fast-running water. Valens crouched and took the box of fire-making tools from his belt. He struck sparks from the flint and steel into the tinderbox. When they caught, he lit his torch. Aulus lit his from that of Valens.

Standing, they could see the huge extent of the cavern. The flickering light of the torches barely illuminated the vault of the roof. Long stalactites hung down, and there were galleries opening in the rock high above. To their left, a shadowed chasm yawned in the floor. From there issued the roar of rushing water. The gloomy vastness reminded Valens of the interior of a temple.

A shelf, perhaps thirty paces long and some five wide, led past the abyss to an outcrop which partly masked the mouth of another tunnel running deeper into the mountain. Thank the gods they had lit the torches. Without them, one misstep and they would have plunged into the void.

Suddenly they were surrounded by dozens of dark winged shapes. Both ducked and waved their torches as the bats flitted, squeaking fearfully around their heads. Equally suddenly, the bats were gone.

‘The air is still good,’ Aulus said. The flames of the torches sawed, the smoke streaming back the way they had come. ‘Further in there must be a shaft leading to the surface.’

Aulus was right; Valens could feel a cold breeze on his face. It came from yet deeper in the caverns. Aulus led the way to the shelf. Above the noises of the water and their own movements, Valens thought that he half heard a chink of metal on stone.

‘Listen,’ he hissed.

Aulus stopped.

They cocked their ears, straining to catch any alien sound.

Another chink of steel on rock. It seemed to come from the galleries above.

Then there was nothing but the subterranean river and the rasp of their own breathing.

Valens was beginning to think the noises were nothing but an echo of their own movements, a trick of the cavern walls, when he heard a low grating sound.

‘Run!’ he shouted.

True to his training, Aulus instantly obeyed.