Swimmers should be cautious of the unseen threat moving quickly beneath the waves this summer, experts say.
No, it's not a shark -- it's the rip currents.
Rip currents, also known as riptides, occur when the water from breaking waves is funneled into narrow channels below the surface, which can often be too fast for swimmers to overcome.
According to the National Weather Service, 57 people died in rip currents across the United States in 2014 -- more than the number killed by tornadoes, lightning strikes or winter storms. Rip currents also caused 26 injuries, and more than $40 million worth of property damage, the weather service reported.
On Friday, Suffolk police rescued three men off Fire Island who had been pulled out to sea by a rip current. Saturday, a Brooklyn man died after being pulled into a rip current near Long Beach -- the first fatality resulting from a rip current on Long Island since 2010, when there were seven fatalities, the National Weather Service said.
Dangerous rip currents aren't restricted to a certain month, season or time of day, and they can form with varying intensity at any beach on a large body of water, the weather service said.
Nelson Vaz, a weather service senior meteorologist in Upton, said beaches on the ocean -- such as those along the South Shore of Long Island -- are especially susceptible.
"You need breaking waves," he said. "The higher the wave, the more water that's being pushed toward the shore."
On its way back out to sea, that water is sometimes blocked by a sandbar, or by man-made structures such as piers, which force the water to funnel into small channels that can overpower swimmers.
Vaz said a rip current can travel as far out as 100 yards, if not farther.
But it's not always clear even to meteorologists and lifeguards when and where a rip current will develop, so there are measures swimmers should be prepared to take if they find themselves caught in one.
Pete Wichrowski, another weather service meteorologist, recommended not fighting against the current, because it will usually result in exhaustion.
"Even expert swimmers have a tough time," Wichrowski said. "Or they can't fight it at all."
Instead, swim parallel to the coastline, because rip currents can be fairly narrow, said Jack Boston, an AccuWeather expert senior meteorologist.
"The best advice is you shouldn't go out where the water is over your waist at all," he said. "Especially if there are warnings of riptides."