A professional diving instructor on Monday warned about the risk of blackout after a recreational diver nearly drowned at Divecube Hotel (潛立方旅館) in Taichung’s Situn District (西屯) on Sunday.

The 29-year-old man, surnamed Chang (張), was reportedly practicing free diving to a depth of 11m in the hotel’s 21m-deep pool shortly before becoming unconscious and sinking to the bottom.

An on-site instructor reportedly rescued Chang and performed cardiopulmonary resuscitation before paramedics arrived. Chang was receiving critical medical care at a hospital as of Monday.

A diving instructor surnamed Lai (賴) said that free diving — considered the world’s most dangerous extreme sport after skydiving — refers to diving without the aid of breathing equipment.

Diving without an external source of air leads to a higher risk of blackout from a lack of oxygen, which is exacerbated when diving alone or with a partner untrained in first aid, he said.

Some free divers are attracted to the practice as it eliminates air bubbles that could disturb fish during a dive and does not require lines and equipment, he added.