The Hawaiian waters were crystal clear and even at a hundred feet below the surface the visibility was excellent.  Astrid looked down at the undersea dome, suspended on massive cables from the ship above, as she dove her mini-sub in a sweeping downward spiral.  She noticed a shadow passing her just as the radio popped to life.

“Astrid!” called Toby.  “Look up.”

The girl inventor leaned forward and looked up through the Plexiglas canopy of the mini-sub, to see the massive form of a humpback whale swimming above her.  The great beast paused for a moment and rolled over onto its side as though giving the strange man-made devices a good once-over, and then with a kick of its great tale, it swam away.

“That makes you feel tiny, doesn’t it?” said Toby.

“It sure does.  I didn’t expect to see any humpbacks.  Their season is just about done.”

They continued descending until both mini-subs reached the bottom edges of the dome, which thanks to preplaced guide wires, now rested snuggly in place on its foundation.  Astrid flipped on the external spotlight, even though at this depth it was still just light enough to see.  Using the manipulator arms, she withdrew a large nut from a container on the foundation and threaded it onto a massive bolt sticking up from the concrete through a hole in the dome’s titanium frame.  She did this twice more and then maneuvered to the next attachment point.  Toby did the same thing, starting on the far side of the dome, and between them they connected all twelve points.

“This thing is huge,” said Toby over the radio.

“It does look pretty giant down here, at least from the outside.  I’m wondering how big it’s going to seem from the inside.”

“All right.  Shall we go on up?”

“Let’s.”

They began spiraling upward, their propellers leaving a double helix of bubbles in their wake.  Halfway to the surface, a shadow passed overhead, and Astrid looked up expecting to see their humpback whale once again.  It wasn’t the whale though.  It was another submarine.  This craft was not a huge vessel like a US Navy sub, but it wasn’t a small one man sub like Astrid’s either.  It was about fifty feet long, longer than the humpback, but not quite as big around.  It was painted flat black, making it difficult for anyone on the surface to see it.  Suddenly as she watched, the unknown submarine tilted its bow down.

“What is that guy doing?” she said aloud.

“What was that, Astrid?” called Toby.

“Toby!  Look out!”

She realized what the strange submarine was doing, but it was too late to do anything about it.  The dark vessel’s turn had sent it on a collision course with Toby’s mini-sub.  Astrid could hear the loud crunch as the nose of the black undersea boat smashed into the little yellow vehicle.  The mini-sub flipped over as a huge cloud of bubbles burst from it and shot toward the surface.

Astrid steered toward the other two submersibles, watching in horror as Toby’s slid sideways toward the ocean floor.  The dark-colored sub accelerated away into the fading light.  Toby’s propeller came to a stop and the little vehicle finally came to rest in the silt on the bottom.  The girl inventor maneuvered next to him and, using her vessel’s manipulator arms, flipped him right side up.  Astrid’s heart leapt into her throat.  The Plexiglas canopy of Toby’s mini-sub had been cracked and water was quickly filling up the compartment.

“Toby, can you hear me?” she radioed, but she almost immediately realized that he couldn’t.  The incoming water must have shorted out the electrical systems.  That meant no radio and no power.

“Astrid to Toronto,” she called the construction ship.  “Toby’s in trouble.  His sub is disabled.”