"Perhaps it's steam," suggested Jack.
"Maybe it is," agreed the professor. "If this is a volcano, with lava in it, the water of the ocean, pouring in on the other side, may be changed to steam."
"Do you suppose this hole leads to the centre of the earth?" asked Mark. "I've read somewhere, that the earth is hollow."
"Some scientists believe it," commented the professor. "This looks like a big enough hole to lead clear through to China. Hark, you can hear the roar of the water now."
They listened, and the wind brought to them the sound of the sea pouring down into the unfathomable depths.
"Let's throw a big rock down," suggested Jack. "Maybe we can hear it strike bottom."
With the aid of Mark he cast a big boulder down into the depths. They listened intently, but not the slightest sound echoed back.
"I guess the bottom is too far away for you to hear the stone land," said the professor.