"Is the ship damaged?" asked Mark.
"I think not," replied Mr. Henderson. "But we are sinking. Look at the depth gage."
The hand on the clock-face was moving slowly around. From ten it went to twenty feet, then to thirty and kept going until it stood at seventy.
"Look to the air tanks," ordered Mr. Henderson to Washington, who, by this time had recovered from his fright. "See if they are all right."
The colored man came back in a few minutes and reported that the supply of compressed atmosphere was safe and that there was plenty of it.
"That's good," remarked Mr. Henderson. "Whatever else happens we can breathe for a while."
"But what has happened?" asked Andy.
"I think the top part of an iceberg toppled down on us," was the reply. "You know about nine-tenths of a berg is under water. Sometimes there is a warm current of the ocean underneath the ice, and it melts. Then it becomes top-heavy and tilts over. One of that sort must have caught us, and has shoved us down into the sea."
"But why don't we rise again when the ice floe slips off us?" asked Mark.
"Because, in all probability the ice will not slip off us," answered the professor grimly. "It may be so large that it has caught us like a bug under a barn door."