You're likely thinking of rip currents. These are powerful, narrow channels of water that flow outward from the shore, perpendicular to the coastline. They are often formed when waves break at different rates, creating a "rip" in the wave pattern.

Here's why they're dangerous:

* Strong pull: Rip currents can pull even strong swimmers away from the shore.

* Difficult to fight: Trying to swim directly against the current is exhausting and often ineffective.

* Panic: The feeling of being pulled out to sea can be overwhelming and lead to panic.

What to do if caught in a rip current:

* Don't panic: Stay calm and conserve energy.

* Swim parallel to the shore: Swim horizontally away from the current until you are out of its pull.

* Call for help: If you can't escape the current, wave your arms and call for help.

Remember, rip currents are a serious hazard, but if you know what to do, you can increase your chances of staying safe.