At what point should a diver turn and begin their return to the exit?

The appropriate time for a diver to turn and start their return to the exit is when the first diver has used one-third of their gas supply. This practice ensures that divers maintain a safe reserve of gas for ascent and any potential issues during their return journey.

Using one-third of the gas supply allows the remaining two-thirds for the return trip, which is essential for managing unexpected circumstances such as a longer-than-anticipated return time or the need for additional safety stops. It also takes into consideration various factors like changes in swim speed or current, which could further deplete air supply.

Turning at this point also helps divers avoid situations where they could become dangerously low on gas as they ascend, ensuring a safer overall diving experience. This principle is part of the "one-third rule," which is a widely accepted guideline in diving practices, promoting safety and preparedness during underwater exploration.