Jake recorded their heartbeats on the boat. Heartbeats, from a shell. Jake was a dreamer, a genius with a terrible temper, and as hard as the shells they tagged. He knew more about the motor systems of Strombus gigas than anyone on earth, but with the funding they had, his project was going to fold unless conch chowder suddenly turned out to be a cure for AIDS.

Now he wanted Mike to use their last BPST. Mike wondered whether the nearby wreck wouldn't distort the signal, but Jake knew acoustics better than he did, so he didn't question him.

He gently picked up the biggest conch. He squirted a glob of marine epoxy onto a flat spot on its shell and slapped a BPST in place. He pressed it down with his thumb and held it for five seconds. He hoped the conch, with his crazy leaping, wouldn't buck it off.

He tagged the rest of the colony with simple metal plates, which had the project's address on them. When he was through, he looked at his watch. He had ten minutes of bottom time left before he would risk the bends.

A brilliant green parrot fish flashed by. Another barracuda slithered past.

As always, he hated to leave the bottom before his time was up. Down here was a world he loved and understood. He was a part of it. For a moment he cavorted around; he was a creature of the sea, soaring and swooping through reefs and valleys, flying in a turquoise sky.

In a surge of power from his flippers, he swam off toward the wreck.

***

Neptune's Folly was tied to a barge permanently anchored in Masthead Bay. In her deckhouse, Jake McCay swore gently. He hunched closer to the loudspeaker, listening to the heartbeat of their latest conch: Bop-bop ...bop-bop... bop-bop...

Something was wrong. He flipped a switch, displaying the signal on the screen of his old DEC computer.

He saw twin peaks rising, then two more, then a range of them marching, out of step, across the CRT. A T wave, a CRW, a P...

Unless this conch needed open-heart surgery, he was getting an echo he didn't want.

The heartbeat was useless. He knew instantly that he never should have made Michael waste their last transmitter so close to the wreck of the freighter.

There was nothing wrong with the BPST. The device, the pride of his heart, was doing its job. And the laws of physics were doing theirs. Water was a wonderful medium for sound: the acoustic waves from the conch's tiny transmitter were traveling beautifully, moving at four times the speed of sound in air, with nearly five times the range.

Picked up through a transducer near the keel, they told him that the conch was a large one, probably male.