He had expected something of this sort.
When he had topped the ladder and found himself in a chamber that stretched away into blackness, he grunted again his mental confirmation of a theory working out beautifully in fact.

His candle held close to the wall, he moved forward along the well-trodden path, looking for a door.

Mechanically he noticed also the formation of the wall and the vein of ore--probably high-grade in pockets, at least--that had caused this chamber to be dug.

The ore, he judged, had long since been taken out and down through the stope into the tunnel and so out through the main portal.

These workings were old and for mining purposes abandoned.
But just now Casey was absorbed in solving the one angle of the mystery which he had stumbled upon at first, and he gave no more than a glance and a thought to the silent testimony of the rock walls.
He found the door, fastened also on the outside just as he had expected it would be.