For a while afterwards this cave had been visited by the curious.  All this smuggling, however, was now a thing of many years past, and curiosity-seekers had come to leave the place alone.

M. Lemaire, however, in studying the surrounding country, had heard of the artificial cave.  He visited it.  At need, he saw that it would suit his purposes.  And now Jack Benson lay there, having been brought hither in Mlle. Nadiboff’s automobile.

The young submarine captain was now not gagged.  He had yelled for help perhaps two hundred times in the long hours since his enemies had left him there.  Yet there had been no response.  Benson was now willing to believe that there was now no likelihood whatever of his being able to summon help.

Unable to consult his watch, and lying there in complete darkness, the submarine boy had lost track of time.  It was now nearly two in the morning.  He had not eaten since early the morning before.  He was famished, and, what was much worse, was parched for want of a drink of water.

“I wonder if they intend to leave me here to die?” thought Jack Benson, for perhaps the five-hundredth time.  “It would be fiendish.  Yet looking for mercy in Lemaire would be like looking for a lake of pure water in the Sahara.”

Jack shifted, as much as the chain at ankle would permit.  He groaned with the discomfort of it all.

As if in answer there came another groan, low, hollow, yet unmistakable.  Captain Jack raised himself on one elbow, listening keenly.  The groan was repeated.

“Who’s there?” he called.

By way of answer there came still another groan.  It was hollow, gruesome, and suggested the grave itself.  But Jack Benson was a healthy, intelligent boy, with sound digestion and well tuned nerves.

“If you’re trying to work any ghostly trick on me,” called Benson, derisively, “try something else!”

Again the groan, a bit louder, but Jack’s answer was a merry, ringing laugh, in which there, was not a trace of dread.

“Thank you for the company, Mr. Groan,” he called cheerily.  “I was beginning to feel a bit lonely.  But say!  Can’t you bring a light—­even a ghostly one?”

“I am the spirit of Paul Jones,” breathed a low, wailing voice.

“Oh nonsense!” jeered Jack.  “Paul Jones never spoke with a cheap French accent.”

“I come to—­to warn—­you,” sounded the same sepulchral accents.

“Bring the warning right in and let’s have look at it,” begged Jack, heartily.  Some convulsive sobs sounded out by the passageway.

“Oh, say,” chuckled Jack, “as a vender of blood curdling noises you’re in need of repairs.  Listen!  I’ll sound a much better line for you!”

With that, and in a deep, blood curdling voice, Captain Benson started in on the first verse of “Down among the dead men.”

He was interrupted then by a more tangible sound.  Beyond, a match was scratched.  Then a lantern was thrust in from the low tunnel, followed by the appearance of the rather long body of Gaston, the chauffeur.