Hundreds of swimmers were rescued yesterday from strong rip currents and high surf at L.A. beaches.

Lifeguards rescued more than 400 people from the waters of L.A. County beaches on Tuesday as dangerous rip currents and high swells caught many swimmers and surfers off guard, according to KTLA. Rescue boats from Baywatch Santa Monica and Baywatch Del Rey conducted a "blitz" rescue at Venice Beach, pulling as many as 18 people from the water at the same time in a large-scale operation. One witness described the chaotic scene of the mass rescue for the struggling swimmers, "Within minutes of seeing that, the lifeguards kind of went on high alert and all of a sudden there was probably 10 to 15 lifeguards in the water in two rescue boats."

Rip currents are difficult to spot from the shore and can easily pull even the strongest swimmers out to sea. L.A. County Lifeguards warn that strong rip currents are expected to continue through Friday, and advise swimmers and surfers to stay near open lifeguard towers. Increasing heat and humidity forecasted to hit Southern California is expected to bring many more people to the waters along the coast this week. Officials anticipate that many more rescues may have to be conducted before the week is over. Lifeguard Lydia Barillas tells KTLA, “Probably similar to today, this was a busy day, and we’re expecting—because of the warm weather and continuing south swell—we’re expecting [the] same kind of activity, and our beaches are completely fully staffed.”

For those willing to brave the waters, Barillas advises, “Please swim in front of an open lifeguard, and ask questions, ask the lifeguard where the hazards are, where it’s safe to swim. And use your best judgement, and stay off any rocks and jetties.”

Rip currents account for roughly 80% of all rescues made by lifeguards, according to the L.A. County Fire Department, though many swimmers are unaware of the serious hazard they pose. Rip currents form at breaks in sand bars and near jetties and piers, and can easily catch swimmers and surfers off guard. The strong currents can reach an average speed of 1- to 2- feet per second, and reach up to 8 feet per second, faster than an Olympic swimmer, according to the National Weather Service.

If swimmers find themselves being pulled away from shore by a rip current, they are advised to not fight the current or swim directly to shore, according to the L.A. Times. Instead, swimmers should paddle parallel with the shoreline to break free from the current's pull and then swim at an angle away from the current and towards the shore.