You know what danger looks like: a whirling saw blade. An enraged pit bull lunging against a chain-link fence. A fallen electrical cable, bare metal sparking on wet pavement: Some kinds of danger are so overt that they seem to radiate a palpable sense of menace. Just to be near them is to be on high alert.

Evidently some dangers aren’t obvious enough, though. Even in an age in which EMTs and emergency room doctors are capable of saving all but the most grievously injured, accidents kill 120,000 Americans a year. It’s the third most common cause of death among American men between the ages of 18 and 65.

It’s tempting to label accident victims as careless or stupid, the losers in life’s ongoing game of Darwinian selection. 'I would never do that,' we tell ourselves, when we hear about someone else’s fatal error. Believing that is a way of insulating ourselves from the reality of life’s terrifying fragility. In fact the world is filled with all kinds of dangers, and many of them aren’t as obvious as sparking cables or whirling blades. Perfectly sane and reasonable people fall victim all the time.

What makes some potentially lethal threats sneakier than others? It has to do with the way the brain processes danger. Primitive circuitry nestled deep within the folds of the cerebral cortex constantly monitor sensory data streaming in from the eyes and ears, looking to match patterns that are known through instinct or learning to be associated with danger. “We interpret external cues through our subconscious fear centers very quickly,” says Harvard University’s David Ropeik, author of How Risky Is It Really? That prickly sense of danger we feel, Ropeik says, is actually our subconscious mind alerting us that it’s detected something in the environment that fits one of these templates.

The problem is, the subconscious mind lacks subtlety. It can’t parse the meaning of complex or unfamiliar scenarios. If it doesn’t detect its preset triggers, it will hum along quietly in the background as usual. If you’re waiting for alarm bells to sound to alert you to every lurking threat, then, you could walk right into a death trap.

In my cover story for this month's Popular Mechanics, I look at 20 kinds of hazards, both natural and man-made, that can cut short a life with little warning. The trick to avoiding them, I suggest, is to move beyond instinct and bring your conscious, rational mind to bear. By learning about about life’s hidden hazards beforehand, you’ll be able steer around danger even without the inner alarm bells going off. And that’s no mean feat. Statistically, one in seven American men will end his life prematurely thanks to an accident. Forewarned, you should be able to stretch those odds considerably.

One example of a death trap is a type of river feature called a low-head dam. Compared to the roaring tumult of whitewater, the smooth passage of water over a low man-made barrier seems benign, almost inviting. Found on many small or moderate-sized streams and rivers all over the world, these structures can be used to regulate the flow of a river, aerate the water, or prevent invasive species from swimming upstream. But watch out. “They’re called ‘drowning machines’ because they could not be designed better to drown people,” says Kevin Colburn, National Stewardship Director of American Whitewater.

Since so-called low-head dams present to the eye a single line of flat reflective water, they can be very hard to spot when approaching from upstream. Then, once you’re in one, the sheer immutable laws of physics set in. Water spilling over the top of the dam is carried by its own momentum to the bottom, leaving a “hole” at the surface that downstream water rushes to fill. The result is what Colburn calls a “tornado lying on its side,” a ceaselessly spinning cylinder of water that can rip off lifejackets and crush bass boats. The ferocity of low-head dams is so out of keeping with their appearance that even emergency personnel can fall victim. Even though most streams and rivers are well marked with signage warning about the presence and danger of low-head dams, paddlers continue to fall into their clutches, with horrific consequences. In September 1975, two rafters were carried over a dam on the Susquehanna River in Binghamton, New York and caught in the backflow. The fire department launched a rescue boat that promptly capsized, drowning one firefighter. The next day, while trying to recover the body, another boat carrying three firefighters flipped, and when another rescue boat arrived it sank, too. All told, three men died.

Unfortunately, this kind of accident is not rare. About 70 people a year die in dam-related whitewater accidents. The best advice for avoiding a similar fate: Don't go near a low-head dam. If caught inside the hydraulic, don’t try to stay above the water. Instead, curl up and drop to the bottom. Don’t resurface until you’ve passed the churning “boil line” where downstream water is drawn upstream. “It’s a counterintuitive thing to do, and scary to do, but the only outflow is at the bottom of the river,” says Colburn.

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