An excerpt from the Prologue:
When we reached the spot and unearthed the little box the instrument was quiet, nor did repeated attempts upon the part of our telegrapher succeed in winning a response from the other end of the line. After several days of futile endeavor to raise Pellucidar, we had begun to despair. I was as positive that the other end of that little cable protruded through the surface of the inner world as I am that I sit here today in my study— when about midnight of the fourth day I was awakened by the sound of the instrument.
Leaping to my feet I grasped Downes roughly by the neck and dragged him out of his blankets. He didn't need to be told what caused my excitement, for the instant he was awake he, too, heard the long-hoped for click, and with a whoop of delight pounced upon the instrument.
Nestor was on his feet almost as soon as I. The three of us huddled about that little box as if our lives depended upon the message it had for us.
Downes interrupted the clicking with his sending-key. The noise of the receiver stopped instantly.