I started upon hearing the low whisper of my companion at my ear.

“In the bay yesterday the Signor saved my life.  I give the Signor, in return, my life’s secret.”

He seized my right hand in his left with a sinewy clutch, and pointed a stiff finger at the luminous blots.

“See there, and there, and there,” he shrilled.  “One floats and wavers like a spineless ribbon of seaweed in the water; another burns with a steady radiance; a third blares from its fissure like a flame driven by the blowpipe.  It is all a question of the under-draught, and some may feel it a little, and some a little more or a little less.  Ah! but I will show you one that feels it not at all—­a hole, a narrow shaft that goes straight down into the pit of the great hell, and is cold as the mouth of a barbel.”

The bones of his face stood out like rocks against sand, and the pupils of his maniac eyes were glazed or fell into shadow as the volcano lightnings fluttered.

Suddenly he drew me to a broken pile of sulphur rock lying tumbled against a ridge of the mountain that ran towards the crater.  It lay heaped, a fused and fantastic ruin; and in a moment the old man leapt from me, and was tugging by main strength a vast fragment from its place.

I leaned over his shoulder, and looked down upon the hollow revealed by the displaced boulder.  It was like the bell of a mighty trumpet, and in the middle a puckered opening seemed to suck inwards, as it were the mouth of some subterranean monster risen to the surface of the world for air.

“Quick! quick!” muttered Paolo.  “The Signor must place his ear to the hole.”

With a little odd stir at my heart, I dropped upon my knees and leaned my head deep into the cup.  I must have stayed thus for a full minute before I drew myself back and looked up at the old mountaineer.  His eyes gazed down into mine with mad intensity.

“Si! si!” he whispered.  “What didst thou hear?”

“I heard a long surging thunder, Paolo, and the deep shrill screaming of many gas jets.”

He bent down, with livid face.

“Signor, it is the booming of the everlasting fire, and thou hast heard the voices of the damned.”

“No, my friend, no.  But it is a marvellous transmission of the uproar of hidden forces.”

He leapt to the shallow pit.

“Listen and believe!” he cried; and funnelling his hands about his lips, he stooped over the central hole.

“Marco!  Marco!” he screeched, in a piercing voice.

Something answered back.  What was it?  A malformed and twisted echo?  A whistle of imprisoned steam tricked into some horrible caricature of a human voice?

“Paolo!” it seemed to wail, weak and faint with agony. “L’arqua, l’arqua, Paolo!”

The old man sprang to his feet and, looking down upon me in a sort of terrible triumph, unslung a water-flask from his belt, and, pulling out the cork, poured the cold liquid down into the puckered orifice.  Then I felt his clutch on my arm again.