One night during the early 60s, two divers from underwater demolition team 15 were making their way through the Inky darkness and the water off the coast of Vietnam.

The two Navy divers were on a hydrography mission, meaning that they'd been assigned the task of surveying the ocean bottom along coastal bays and harbours all along the Vietnamese coast.

Even this early, Vietnam was teetering and that put all of the dominoes behind it at grave risk.

The United States might have to do an amphibious invasion and not only would the landing beaches have to be mapped and cleared,

it was also critically important to know the depth and layout of the harbours all along the coast.

Deploying from a rubber raft offshore, this night's mission meant a relatively short swim, maybe five or six miles tops, and all of it underwater, of course.

They were wearing a new re-brither system, which chemically scrubbed the exhaled CO2 rather than releasing it as a scuba tank would.

Now these re-brithers were rare, expensive, and occasionally temperamental compared to a scuba tank in a regulator,

but the Navy was willing to put up with all of that in exchange for the re-brithers one big advantage. They did not leave a bubble trail.

The water was relatively warm that night, but both men were wearing full wetsuits mostly to protect them from the occasional swarms of box jellyfish,

which were utterly invisible at night and whose long trailing tentacles contained a particularly nasty toxin that could incapacitate or even kill men in even that kind of peak physical condition.

The waters of Southeast Asia are also perennially ranked as some of the most dangerous in the world with regard to sharks, including carcarrot-on, freaking carcarius, or the great white shark that could grow to the size of both men lined up head to tail.

Now this particular night was memorable because of a relatively rare phenomenon known as bioluminescence.

At certain times, in certain conditions, immeasurably vast numbers of small marine plankton will glow like billions of microscopic fireflies, although they produce an electric blue color even more striking than the yellow green of the more familiar lightning bugs.

The water soon became alive with billions of these plankton and sudden motion would cause them to fluoresce meaning that the slow, steady strokes of the divers' legs left both men trailing what looked like a fantastic blue plasma.

It was absolutely beautiful and absolutely dangerous, since the last thing the UDT guys wanted was to be outlined in bright blue as they tried to sneak right up into an inhabited active enemy harbor.

Now needless to say they were swimming blind, but that wasn't a problem for these two men from UDT 15.

The two divers were connected by about 10 feet of regular clothesline. One of the divers kept close watch on the dimly glowing compass strap to his wrist while his partner did the same thing with his depth gauge.

One man constantly monitored in direction and the other paying attention to depth.

If the guy with the compass started to swim too shallow or too deep, he would feel the tug of his buddy through the clothesline, same thing would happen if the other fellow started to drift left or right off of course.

Now as they approached their target and the water became shallower, the blue flashes of feeding fish grew more frequent and more pronounced.

And then in an instant, a blue wall of light lit up on their left, a breathtaking huge flash of bioluminescent light.

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