nging downward. At a depth of
twenty feet the angry motion of the waves was unfelt. In those dim
depths all was as quiet and undisturbed as if the elements were at
perfect peace above.

Down, down dropped the submarine till her depth indicator showed that
she was submerged five hundred fathoms.

"The chart gives seven hundred hereabouts," commented Ensign Hargreaves,
glancing at it; "so I guess we are safe for forty miles more before the
floor of the ocean slopes upward. We must go up a bit higher then."

The inventor nodded.

"I understand," he said, and then, "we are now running at what speed?"

The ensign turned to the speed indicator.

"A trifle under twenty miles an hour," he said.

Mr. Barr glanced at the clock before him, which was illuminated by a
tiny shaded electric bulb.

"I'll keep on this course at this speed for about two hours then," he
determined.

"That will be all right, I imagine," was the rejoinder, "but don't keep
on too long. The bed of the sea, according to the chart, rises up very
rapidly further on. It must be almost cliff-like in its sudden
elevation."

"I'll be on the lookout," the inventor assured him.

Rob descended the ladder once more and reentered the engine room to find
out how Merritt was getting along. He found the young engineer seated on
the leather lounge alongside the engines watching them lovingly.

"Work smoothly, don't they?" he said.

"They sure do," was the other's response; "smoothly as a Geneva watch."

The boys sat chatting on various matters, and the time flew along
rapidly till Rob suddenly looked at his watch.

"Almost two hours. It's time we were rising," he said.

"What do we want to rise for? It's deep enough here, isn't it?"

"That's just it. The ensign says that the chart shows that a sort of
submarine cliff looms up right ahead of us somewhere hereabouts."

"Great ginger snaps! I thought the bottom of the sea was as level as a
floor."

"Not a bit of it. It's as full of mountainous regions and flat,
depressed plains and valleys as the Rockies themselves."

"Gee whiz! I'd hate to hit one of them. I----"

Merritt stopped short. A terrific crash shook the submarine from stem to
stern. Rob saved himself from falling into the machinery by seizing a
rail.

For an instant the vibration lasted, and then the diving craft came to a
dead stop.

The boys gazed at each other with blanched faces.

Did the crash mean that they had actually struck one