Beneath the Borderlands
Holding One's Breath
Characters can normally hold their breath for one round (60 seconds). After this period has elapsed, a CON test is required at the beginning of each round to continue to resist exhaling (adding a degree of difficulty with each subsequent attempt). If underwater, a character who fails such a test must immediately resurface or begin drowning. A drowning character is no longer able to engage in activity of any kind and must make a saving throw each round or die.