"How is that?" asked Mark. "I don't quite understand."
"Because there is, I believe, a big sheet of ice above us, one, say several hundred feet thick. The same thing is below us, between us and the real bed of the ocean."
"But suppose we have to go up to renew our air supply?" asked Jack.
"We can't go," replied the inventor.
"Then we will die."
"Not necessarily. We will steam along until we come to a place where there is no ice above us."
"But I thought you said there was nothing but ice above us now."
"So there is, but I intend to head due south and there, I believe, we will find an open polar sea. If we do my theory will be proved and we will have made a great discovery."
"Forward then!" exclaimed Jack. "Let us strike for the open sea."
The Porpoise began to move ahead. She steamed slowly, for Mr. Henderson realized that he was in dangerous waters. He took his position in the conning tower, and had Jack with him to assist in looking for any obstructions that they might unexpectedly meet.