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Anyone who has had the experience of the earth shaking them from a sound sleep, heard the howl of a hurricane ripping off their roof, or poked their head out to see what’s standing after a twister has sent little girls and dogs hurling through the air, knows nature can be deadly.

But few of us stop to consider the lethal potential in a calm patch of ocean water, even though rip currents—often called drowning machines—kill a number of people in the United States on average every year. According to the United States Lifesaving Association, approximately 100 people each year drown at the beach. That’s far more than are killed in an average year by earthquakes, hurricanes, or tornadoes. And the odds of death escalate when there is no lifeguard in sight. In 2008, according to the United States Lifesaving Association, rescuers pulled 71,983 panicked swimmers to safety—over half of them being pulled out to sea by dangerous rip currents. The odds a person will be killed by a rip current in a year are 1 in 6,947,000.

Rip currents are often called riptides or undertows, but both are misnomers. The dangerous currents are not caused by tidal action, but by underwater channels which form along sandbars and other low points near the shore and force a huge volume of water to be swiftly funneled from the beach back towards the open sea. And although it is often impossible to stay upright in the fast-flowing current, the primary force exerted on a swimmer does not pull them under—it carries them away.

Fighting against the current can quickly overwhelm even strong swimmers, and, tragically, family members or heroic strangers often die in a rescue effort. On April 25, 2009, Charles Schulze, a Washington DC attorney, plunged into the ocean when he spotted two boys floundering in the surf off Pompano Beach in Florida. He pulled the 12-year-old boy to safety, and went back for his 9-year-old brother. Schulze managed to get the young boy close enough to land to be grabbed by onlookers, but the effort cost Schulze his life.

The currents are not unique to oceans—they can occur anywhere there are breaking waves, and the bigger the waves, the more dangerous the currents, although the currents themselves are usually found in nearby patches of calm. Every year swimmers drown in rip currents in the Great Lakes, and deadly currents are even common along the beaches that ring New York City. Rip currents have been measured moving as fast as 8 feet per second—outstripping the pace of Olympic swimmers.

The secret to survival is to suppress the instinct to swim to shore. Instead of fighting the current, swimmers are advised to swim parallel to the shore. Rip currents can be 50 feet to 50 yards in width, but eventually a swimmer can break free of its grip.