Year after year, the ocean's most successful killer is not the great white shark.
It's not the deadly jellyfish.
Not even monster waves or hurricane-force winds.
Your worst ocean nightmare during a day at the beach is more likely to be a rip current, experts say.
Every year more than 100 beachgoers drown in these strong rushes of water that pull swimmers away from the shore.
And that's just in the United States.
Nearly half of all rescues made by lifeguards at ocean beaches are related to rip currents, according to the United States Lifesaving Association.
Sharks typically kill about 6 people a year globally.
A common perception is that rip currents pull you underwater, but in reality they're roughly horizontal currents that gradually suck you further and further from the beach.
A rip current near Melbourne, Fla., after Hurricane Jeanne (NOAA)
Here's how they originate: Waves break differently at different parts of a shore — in some places the waves are strong and in others they are weak.
These differing conditions carve out channels in sand bars that lie just off the beach.
When water returns to the ocean, it follows the path of least resistance, which is typically through these channels.
This creates a strong and often very localized current capable of sweeping unsuspecting swimmers out to sea.
The currents usually move at one to two feet per second but stronger ones can pull at up to eight feet per second. (On a track, Olympic sprinters cover about 34 feet per second.)
Heavy breaking waves can trigger a sudden rip current, but rip currents are most hazardous around low tide, when water is already pulling away from the beach.
Hurricanes, widely spaced swells, and long periods of onshore wind flow can also drum up stronger than normal currents.
These conditions also create larger waves, which sometimes draw more people into the water.
What to do
It is easy to be caught in a rip current.
Most often it happens in waist deep water, experts say.
A person will dive under a wave, but when they resurface they find they are much further from the beach and still being pulled away.
What they do next can decide their fate.
Those who understand the dynamics of rip currents advise remaining calm.
A rip current is like a giant water treadmill that you can't turn off, so it does no good to try and swim against it.
The National Oceanic and Atmospheric Administration (NOAA) and the United States Lifesaving Association (USLA) suggest trying to swim parallel to the shore and out of the current. Once you've gotten out of the current, you can begin swimming back to shore.
However, if it is too difficult to swim sideways out of the current, try floating or treading water and let nature do its thing.
You'll wash out of the current at some point and can then make your way back to shore.
If neither of these options seems to be working for you, continue treading water and try to get the attention of someone on shore, hopefully a lifeguard.
The NOAA also emphasizes anyone planning to swim in the ocean should learn to swim well and never swim alone.
Pick a beach with a lifeguard if you don't feel comfortable with your swimming abilities but still want to enjoy the surf.
And finally, take a look at the water -- if it looks dangerous, don't even try it.