Researchers are warning that many people in the Netherlands underestimate the risks of swimming in open water, where hidden hazards and changing conditions can quickly turn a swim into a life-threatening situation, NOS reports. Half of the Netherlands is under a code yellow heat warning on Sunday, with the warm conditions expected to persist until at least Tuesday.

Overall, the Royal Dutch Lifesaving Society advises people to learn how to swim well, never go alone, and stick to supervised locations where swimming is permitted. People who get into trouble should first float on their backs or use something to stay above the water. It is very important not to panic while still trying to call or wave for help.

Unlike swimming pools, lakes, rivers, and other inland waterways often have unpredictable conditions. Swimmers may not know what is beneath the surface, drowning researcher Joost Bierens told NOS. “Pits, mud, stones, plants, and animals. It can also be much shallower than expected, or there may be an invisible current,” Bierens said.

Swimming lessons usually takes place at the pools, so swimmers often enter open water unprepared. “There is no moment in the entire upbringing where the risks of swimming in open water are properly explained,” said Bernard Korte, director of the Dutch Institute for Safe Swimming Locations (NIVZ).

Korte said swimmers frequently overestimate their abilities and fail to consider factors such as wind, waves, and currents. A swimmer who travels with the wind behind them may struggle on the return journey when facing the wind, he said.

“With cycling, people do take that into account, but with swimming it is still an unknown risk,” Korte told NOS. “It has a bit to do with culture; we think we can all swim once we have earned our swimming certificates. Then you learn technique, but not in open water.”

Bierens said panic is one of the biggest dangers when swimmers encounter unexpected problems. “If you get scared and you don’t know what to do, the panic increases. The recovery time is extremely short. To regain control of the situation, you need to get your head back above water, and if you are not used to that, you are helpless,” he said.

Researchers also warn that warmer weather may increase drowning risks as more people seek cooling in lakes and rivers. English research found that every 1 degree Celsius increase in average temperature is linked to a 7 percent increase in drowning risk, with men most affected, according to Bierens.

Experts say more research and prevention efforts are needed. Bierens said the Netherlands is falling behind other European countries in studying drownings and called for long-term research into causes and prevention.

“Although we are a water-rich country, the Netherlands is still somewhat behind international developments. It would be very good if we invested more in long-term research. After all these years, we in the Netherlands are still not able to really look at it over a long period,” he told NOS.