SCHLAAK! The pickaxe hit the wall of earth and, sparking on an unseen shard of flint, sank deep into the clay, coming to a sudden halt with a dull thud.

“This could be it, Will!”

Dr Burrows crawled forwards in the cramped tunnel. Sweating and breathing heavily in the confined space, he began feverishly clawing at the dirt, his breath clouding in the damp air. Each greedy handful revealed more of the old wooden planking beneath, exposing its tar-coated grain and splintery surface, which was thrown into stark relief by the combined glare of their helmet lamps.

“Pass me the crowbar.”

Will rummaged in a satchel, found the stubby blue crowbar and handed it to his father, whose gaze was fixed on the wall of wood before him. Forcing the flat edge of the tool into a tiny gap between two of the planks, Dr Burrows grunted as he put all his weight behind it to gain some purchase. He then began levering from side to side. The planks creaked and moaned against their rusted fixings until, finally, they bellied out, breaking free with a resounding crack. Will recoiled slightly as a clammy breeze bled from the ominous gap Dr Burrows had created.

Urgently, they pulled two more of the planks out of place, leaving a shoulder-width hole, and paused for a moment in silence. Father and son turned and looked at each other, sharing a brief conspiratorial smile. Their faces, illuminated in each other’s light beams, were smeared with a war paint of dirt. They turned back to the hole and stared in wonder at the dust motes floating like tiny diamonds, forming and reforming unknown constellations against the nightblack opening.

Dr Burrows warily leant into the hole, Will squeezing in beside him to look over his shoulder. Their helmet lamps cut into the abyss and, to their left, they illuminated a curved, tiled wall. Their beams, sinking deeper, swept over old posters whose edges were peeling away from the wall and waving slowly, like tendrils of seaweed caught in the drift of powerful currents at the bottom of the ocean. Will raised his head a degree, scanning even further along until he caught the edge of an enamelled sign. Dr Burrows followed his son’s gaze until both of their helmet lights fixed on the name ...