"Perhaps," said Frank, "but—"

"Look at that!" shouted the chopper’s pilot.

A narrow cylinder was rising out of the center of the spaceship’s hull. At its apex was a bright yellow light that was winking on and off. Blink, pause, blink-blink, pause, blink-blink-blink.

"Counting," said Clete.

But the next sequence was five blinks, not four, and the one after that was seven blinks. And then the sequence started cycling over and over again: one, two, three, five, seven; one, two, three, five, seven.

"Prime numbers!" said Frank. He shouted at the pilot, "Does this copter have a searchlight?"

The man shook his head.

"Get us back to the aircraft carrier as fast as possible. Hurry!"

The pilot nodded and took the chopper through a wide, banking turn.

Frank looked over at the Russian sub. It was already returning the signal: the first five prime numbers in sequence, cycling repeatedly.

The pilot was wearing a radio headset. Frank shouted at him. "Get the Kitty Hawk to use its searchlights. Tell it to blink out a reply at the ship. The first five prime numbers, over and over."

The pilot relayed the message. It seemed to take forever — with Frank fidgeting through each second — but eventually a large searchlight just below the carrier’s radar antenna started flashing out the sequence.

The yellow beacon sticking up from the lander went dark.

"Could we have said the wrong thing?" asked Clete.

The Seahawk touched down on the flight deck. As the rotor was twirling down, Frank got out, the wind from the blades whipping his hair. Clete followed a moment later. Hunching over, they hustled away from the chopper. The captain, a bald-headed black man of about fifty, was waiting for them just inside the base of the conning tower. "The Russians are still signaling the same thing, too," he said.