They went up the flight of steps for a considerable distance, then they found the passage blocked by a number of great stones.

Stanley uttered an exclamation of disgust.
"It has fallen in," he said.

"No doubt we are near the top of the rock.

Either the staircase was roofed in, or there was a building erected over the entrance; and either the roof or building, whichever it was, has fallen in.

That is very unlucky.