We stood with bitter hearts staring at this unexpected obstacle. It was not the result of any convulsion, as in the case of the ascending tunnel. The end wall was exactly like the side ones. It was, and had always been, a cul-de-sac.

“Never mind, my friends,” said the indomitable Challenger. “You have still my firm promise of a balloon.”

Summerlee groaned.

“Can we be in the wrong cave?” I suggested.

“No use, young fellah,” said Lord John, with his finger on the chart. “Seventeen from the right and second from the left. This is the cave sure enough.”

I looked at the mark to which his finger pointed, and I gave a sudden cry of joy.

“I believe I have it! Follow me! Follow me!”