Controlled emergency swimming ascent (CESA)

A Controlled Emergency Swimming Ascent is how you get to the surface when you run out of air. You swim upward while slowly letting air out of your mouth. This technique is vital because it stops your lungs from expanding too fast as the water pressure drops.

Divers use this when a regulator fails or they hit their reserve limit. Most instructors teach this for depths between 5 m and 30 m. You must maintain a slow, steady rate so that you do not exceed 18 m per minute.

Never hold your breath during the ascent. I saw a student in Koh Tao in 2019 panic and hold his breath while rising. This mistake causes lung overexpansion. Always exhale a continuous stream of bubbles while you swim toward the light.