Pellucidar

military instrument was my first guess; but really there didn’t seem much likelihood that this was the correct explanation, when one took into account the loneliness and remoteness of the spot.
As I sat gazing at my remarkable find, which was ticking and clicking away there in the silence of the desert night, trying to convey some message which I was unable to interpret, my eyes fell upon a bit of paper lying in the bottom of the box beside the instrument. I picked it up and examined it. Upon it were written but two letters:

 D. I.

They meant nothing to me then. I was baffled.
Once, in an interval of silence upon the part of the receiving instrument, I moved the sending-key up and down a few times. Instantly the receiving mechanism commenced to work frantically.
I tried to recall something of the Morse Code, with which I had played as a little boy⁠—but time had obliterated it from my memory. I became almost frantic as I let my imagination run riot among the possibilities for which this clicking instrument might stand.
Some poor devil at the unknown other end might be in dire need of succor. The very franticness of the instrument’s wild clashing betokened something of the kind.
And there sat I, powerless to interpret, and so powerless to help!
It was then that the inspiration came to me. In a flash there leaped to my mind the closing paragraphs of the story I had read in the club at Algiers:
Does the answer lie somewhere upon the bosom of the broad Sahara, at the ends of two tiny wires, hidden beneath a lost cairn?
The idea seemed preposterous. Experience and intelligence combined to assure me that there could be no slightest grain of truth or possibility in your wild tale⁠—it was fiction pure and simple.
And yet where were the other ends of those wires?
What was this instrument⁠—ticking away here in the great Sahara⁠—but a travesty upon the possible!
Would I have believed in it had I not seen it with my own eyes?
And the initials⁠— D. I.⁠—upon the slip of paper!
David’s initials were these⁠—David Innes.
I smiled at my imaginings. I ridiculed the assumption that there was an inner world and that these wires led downward through the earth’s crust to the surface of Pellucidar. And yet⁠—
Well, I sat there all night, listening to that tantalizing clicking, now and then moving the sending-key just to let the other end know that the instrument had been discovered. In the morning, after carefully returning the box to its hole and covering it over with sand, I called my servants about me, snatched a hurried breakfast, mounted my horse, and started upon a forced march for Algiers.
I arrived here today. In writing you this letter I feel that I am making a fool of myself.
There is no David Innes.
There is no Dian the Beautiful.
There is no world within a world.
Pellucidar is but a realm of your imagination⁠—nothing more.
But⁠—
The incident of the finding of that buried telegraph instrument upon the lonely Sahara is little short of uncanny, in view of your story of the adventures of David Innes.
I have called it one of the most remarkable coincidences in modern fiction. I called it literature before, but⁠—again pardon my candor⁠—your story is not.
And now⁠—why am I writing you?
Heaven knows, unless it is that the persistent clicking of that unfathomable enigma out there in the vast silences of the Sahara has so wrought upon my nerves that reason refuses longer to function sanely.
I cannot hear it now, yet I know that far away to the south, all alone beneath the sands, it is still pounding out its vain, frantic appeal.
It is maddening.
It is your fault⁠—I want you to release me from it.
Cable me at once, at my expense, that there was no basis of fact for your story, At the Earth’s Core.

Ten minutes after reading this letter I had cabled  Mr. Nestor as follows:

Story true. Await me Algiers.

As fast as train and boat would carry me, I sped toward my destination. For all those dragging days my mind was a whirl of mad conjecture, of frantic hope, of numbing fear.
The finding of the telegraph-instrument practically assured me that David Innes had driven Perry’s iron mole back through the earth’s crust to the buried world of Pellucidar; but what adventures had befallen him since his return?
Had he found Dian the Beautiful, his half-savage mate, safe among his friends, or had Hooja the Sly One succeeded in his nefarious schemes to abduct her?
Did Abner Perry, the lovable old inventor and paleontologist, still live?
Had the federated tribes of Pellucidar succeeded in overthrowing the mighty Mahars, the dominant race of reptilian monsters, and their fierce, gorilla-like soldiery, the savage Sagoths?
I must admit that I was in a state bordering upon nervous prostration when I entered the ⸻ and ⸻ Club, in Algiers, and inquired for  Mr. Nestor. A moment later I was ushered into his presence, to find myself clasping hands with the sort of chap that the world holds only too few of.
He was a tall, smooth-faced man of about thirty, clean-cut, straight, and strong, and weather-tanned to the hue of a desert Arab. I liked him immensely from the first, and I hope that after our three months together in the desert country⁠—three months not entirely lacking in adventure⁠—he found that a man may be a writer of “impossible trash” and yet have some redeeming qualities.
The day following my arrival at Algiers we left for the south, Nestor having made all arrangements in advance, guessing, as he naturally did, that I could be coming to Africa for but a single purpose⁠—to hasten at once to the buried telegraph-instrument and wrest its secret from it.
In addition to our native servants, we took along an English telegraph-operator named Frank Downes. Nothing of interest enlivened our journey by rail and caravan till we came to the cluster of date-palms about the ancient well upon