When there is a risk of flash flooding, avoid underground areas. This is the lesson that a series of recent floods have highlighted after they caused multiple casualties.

As powerful Typhoon Hinnamnor hit the southern city of Pohang, North Gyeongsang Province, early Tuesday morning, several people went missing in a flooded underground parking lot of an apartment complex. Two were found alive while seven were pronounced dead on Wednesday.

Residents said there were several announcements from the apartment’s management office about the conditions of the complex’s several parking lots between Monday night and Tuesday morning and the last one urged people to move their cars swiftly citing flood risks.

When heavy rains triggered flash floods in Seoul last month, similar incidents occurred.

A man in his 40s went to check his car parked in an underground parking lot in southern Seoul's Seocho District on Aug. 8, only to be found dead days later.

On the same day, in the western Seoul neighborhood of Sillim-dong, a family of three drowned in a semi-basement home, known here as banjiha. Another was found dead in the same neighborhood, trapped in a flooded banjiha home.

Experts warn that once water starts flowing in, it doesn’t take long for the situation in underground spaces to become critical. They say people who happen to be in such places should leave before water levels rise to their shins.

In a 2014 study by the National Disaster Management Research Institute, both men and women failed to move out of an underground space when the stairways were flooded and the water level reached their knees.

While wearing slippers or high heels, water levels of 35 centimeters or above can make walking difficult, the study also found.The researchers said finding an object to hold onto such as handrails can be useful when trying to get out of flooded underground spaces.