How good is the visibility today? It's an absorbing question for a primarily visual animal who's peering at an alien world through a mask. And the usual answer, "There's 60-foot vis" or 100-foot or whatever, is often misleading.

Saying the vis is 60 feet implies you can see adequately and equally for 60 feet in all directions, as if you were the center of a sphere of clarity 120 feet in diameter. That might be the case in air, but water plays such odd tricks with light that a diver's range of vision is shaped more like a soap bubble in a breeze, wavering from ovoid to sausage-shape to foam. Even if you're suspended out in blue water, you're rarely able to see the same distance in all directions, and how far you can see changes from minute to minute.

Despite common dive conventions, it's really impossible to put a single accurate number to the visibility limit because it depends on a number of ever-changing factors.

Let There Be Light

Take that whale shark out at the far edge of visibility. How far away from it could you be and still see it? There's no simple answer because the visibility limit depends on the depth, the time of day, the direction you're looking and what you're looking at. To begin with, consider the amount of light reaching the whale shark and reflected toward your eye--more light means better visibility.a€?It's as simple as that. So you can expect less visibility on a cloudy day than on a sunny one. Even clouds passing in front of the sun temporarily affect the vis.

The position of the sun in the sky also affects the penetration of light into the water. Maximum penetration is at noon, when sunlight strikes the water at 90 degrees. But as the afternoon goes on and the angle of sunlight to water declines, more light is reflected from the water surface, and less penetrates to the shark. In theory, when the angle of sunlight reaches 48 degrees to the surface (a little before 3 p.m. in the tropics), all sunlight will be reflected back to the sky, none will illuminate the shark and under water will be as dark as night. In fact, it's not that dark because the water surface is never totally flat, and the faces of small waves catch the light. Also, the sky itself casts diffused light downward from all directions.

Nevertheless, as a general rule, you can expect visibility under water to increase throughout the morning until noon and decrease again in the afternoon. It will be best by far at noon on a sunny day--even though the water clarity hasn't changed, just the amount of light is different.

Next, how deep is the shark? Every foot of water the sunlight has to pass through absorbs and scatters some of the light, leaving less to illuminate the shark. The amount of absorption depends on the color of the light--blue light penetrates better than red--but in general, only 18 percent of sunlight penetrates 60 feet of water. That could be 20 feet of depth plus 40 feet of distance, or 40 feet deep and 20 feet away, with about the same visibility. So the best visibility is going to be in shallow water, other things being equal. It will be even better if the shallow bottom is white sand that reflects light upward to the shark.

Finally, how reflective is what you're looking at, and what's the background? The apparent visibility will change tremendously depending on whether you're looking at a black wetsuit against a black cave mouth, a silvery fish in sunlight against a black cave mouth or a white fish over a white bottom.

Dirty Water

All those factors--the amount of light, the depth and the contrast to the background--can make the difference between 50 feet of visibility and 200 feet, even assuming clear water. In fact, "clear" water is anything but. That's why we measure underwater visibility in feet instead of miles. To begin with, water is densely packed with molecules, some 800 times more densely than air. To get to your eyes, light has to fight its way through that crowd.

In the ocean, light has more than molecules to contend with because even "gin-clear" seawater is absolutely crammed with stuff. A rule of thumb among marine biologists is that one liter of average clear seawater contains one million single-celled creatures called phytoplankton, another million single-celled protists, a billion bacteria and 10 billion viruses. And that's only the stuff that's alive. Add sand, silt, dust, salt ions and the detritus of those 11-odd billion creatures in every liter around you.

All of it intercepts the light before it can reach your eyes and absorbs it, scatters it and bends images into new shapes. Air hardly slows down light from its top speed of 186,000 miles per second. Crystal-clear water slows it a lot, to 135,000 miles per second. Really soupy water, as we know, stops it dead.

How bad can it get? Zero vis, obviously. Most of us can avoid diving in muddy harbors, and we can avoid kicking up bottom silt, but sometimes what should have been good vis goes bad. A rainstorm that washes dirt into the ocean is one cause, and a plankton bloom is another. Plankton, silt and mud suspended in the ocean absorb and reflect the light passing through it, leaving images dark and unfocused.

In Full Bloom

Plankton is a generic term for many kinds of microscopic water-dwelling creatures, both animal and vegetable. Blooms occur worldwide, says Dr. Linda Rasmussen, a researcher at the Scripps Institution of Oceanography at the University of California at San Diego. Though less common in the tropics, blooms happen on both coasts of the continental U.S., as well as the Arctic and Antarctic. They happen in freshwater lakes as well as in the ocean.

The water temperature, the amount of sunlight and oxygen, and upwelling currents that bring nutrients to the surface affect visibility. Some types of plankton feed on others, while bacteria and viruses both kill and multiply simultaneously. Blooms typically occur in spring and summer as longer days bring more sunlight to the water for photosynthesis. Plant life--the phytoplankton--reproduces explosively, its chlorophyll tinting the ocean green. At the same time, microscopic animal life--the zooplankton--move in to graze on the plants, and the fish arrive to feed on both. Meanwhile, the long, sunny days are warming the upper water, which separates into layers. With nutrients locked below the thermocline, the bloom above it may now starve and fade--until strong winds or currents mix the layers of water, bringing nutrients to the surface for another explosion of eating and being eaten.

Because the thermocline separates populations of plankton, you can sometimes dive below a layer of plankton and find better visibility.

"Locally," says Dr. Rasmussen, referring to Southern California, "I have seen plankton blooms in layers from zero to 20 feet, 40 to 60 feet, and almost anywhere in between, as well as all the way from the surface to 70 feet or more. It is very common here for divers to report where these low-vis layers are after they dive, though it can change from one day or week to the next." While you may find clear water underneath, she adds, the plankton may be so thick that you'll need a dive light because almost no sunlight penetrates.

Blue Haze

There's a final problem with that question, "What's the vis?" No matter how good it is, no matter how clear the water and bright the sun, the maximum distance you can see in the water is almost certainly less than you think. That is only one of the optical illusions created when light bends and twists its way through water. Whatever you see is not what you get. It's actually bigger or smaller than it looks, longer or shorter, closer or farther, redder or greener, or almost anything but what it seems. That we see as well as we do in this fun house is a tribute not so much to our eyes as to our brains' amazing ability to make complete pictures from incomplete data. But sometimes we're fooled in surprising ways.

For example, how far away is that whale shark out at the edge of visibility--100 feet? 150 feet? Unless you can measure the distance, your guess is almost certainly off by a lot, and moreover it's off in the "wrong" direction. You've heard that water magnifies images, making them look bigger and therefore closer than they really are. That's true with things close to you. The lobster you're about to grab is actually smaller and farther away than you think. But at longer distances, and especially in murky water, our brains play a strange trick on us called "vision reversal." Though the whale shark looks bigger than it is, we think it's farther away, not closer.

It results from how your brain guesses distances when most of the normal, dry-land ways of estimating distance, like comparing the sizes of familiar objects receding into the distance, are missing. There aren't many objects of known size under water. There's rarely a foreground or middle distance to locate what you see, just a featureless blue background. Even stereoscopic vision doesn't help much when the whale shark out there is a two-dimensional silhouette.

Lacking the normal distance clues, your brain is left with the "blue haze" factor: because things at a distance in air look fuzzy, indistinct and bluish, your brain equates blue haze with distance. Of course, everything looks fuzzy and indistinct in the water, especially when the water isn't crystal-clear. Still, your brain tries to explain the blue haze by assuming more distance, so you think the whale shark is farther than it is--120 feet, say, instead of 80.

Now the fun house gets funnier. Overestimating the distance causes you to overestimate the size of the shark too. Remember, its image on your retina is still magnified. Your brain says, "It looks big and it can't be close, so it must be huge." As a result, a 20-foot shark 80 feet away may appear 120 feet away and therefore 40 feet long. So you don't have to believe every fish story you hear.

It's not just theory. Vision reversal has been demonstrated in the lab and observed in the field. In the late 1960s, researcher J.A.S. Kinney put divers in water tanks and had them guess distances and sizes. She found they consistently underestimated distances out to three or four feet, and overestimated greater distances. The point of "vision reversal," where they switched from underestimating to overestimating, was farther away in clear water than in murky water, probably because murky water makes even close things look hazy and indistinct.

Richard Roesch, now head of Human Factors Engineering at the Naval Surface Warfare Center in Panama City, Fla., and once a pilot on the Johnson Sea-Link project, has seen vision reversal in the field. His job required maneuvering the submersible and its manipulator arm in close quarters and poor visibility. "It was humorous to watch somebody trying to pick something up for the first time and grossly overestimate the size and distance," he recalls. "They'd go way past the object when they'd try to reach it." Bottom line: The limit of visibility is probably not as far away as you think it is.

Crunching the Numbers

But if you had some kind of measuring tool, how far could you see in clear water? Claims of 100-foot, 200-foot, even 300-foot visibility can be found in travel literature and heard around every dive resort bar. Can these incredible claims be true? Maybe. NASA safety divers have told me they can see from one end of the 202-foot Neutral Buoyancy Lab pool in Houston to the other "as plain as day." But that's a shallow pool of filtered fresh water, under bright overhead lights--as ideal as conditions get. What about real-world underwater visibility, as divers experience it?

It's anybody's guess, but Roesch has a strong entry in the open-ocean division of the visibility sweepstakes. One night in the early 1970s, he was in a chamber on the bottom of the famous Tongue of the Ocean trench in the Bahamas. While locking out to a submersible to return to the surface, he looked up and could see a bright light.

"We thought, a€?This is incredible! From 2,012 feet, this has got to be something in the water.' We had visions of UFOs, another submersible or perhaps a remote-controlled vehicle or something like that. We had no idea what the source of the light was. Until we got to about 1,000 feet. And then we could very clearly see that the source of light was the moon. So I can say that I have seen the moon from 2,012 feet down."