The hole was bigger now, and I got down on all-fours in a channel not much bigger than a large sewer-pipe and found myself in total darkness. A minute later a heavy thud, followed by a cataract of loose sand, announced the arrival of the Colonel.

“Catch hold of my heel,” called Dr. Silence, “and Colonel Wragge can take yours.”

In this slow, laborious fashion we wormed our way along a tunnel that had been roughly dug out of the shifting sand, and was shored up clumsily by means of wooden pillars and posts. Any moment, it seemed to me, we might be buried alive. We could not see an inch before our eyes, but had to grope our way feeling the pillars and the walls. It was difficult to breathe, and the Colonel behind me made but slow progress, for the cramped position of our bodies was very severe.

We had travelled in this way for ten minutes, and gone perhaps as much as ten yards, when I lost my grasp of the doctor’s heel.