It&#x27;s surprising to see such an article #1 in HN ... right after a long weekend of cave diving training!<p>It&#x27;s a sport that requires a lot of preparation, planning, risk estimation and counter-measures. My hobby cave dives are nothing like such explorations, but the principles are the same.<p>In the end, when you go diving, the level of risk is close to 0: one regulator may fail (risk around 1&#x2F;100 if well checked), two won&#x27;t fail during the same dive (risk of two failures 1&#x2F;100 x 1&#x2F;100 = 1&#x2F;10000 which is negligible--if I got the figure right, I didn&#x27;t double check! EDIT: it&#x27;s rather 1&#x2F;1000 x 1&#x2F;1000 = 1&#x2F;1.000.000). And we use two independent air tanks, and with air reserves managed with one regulator fault in mind, as well as the &#x27;little&#x27; panic induce by the regulator fault. That&#x27;s called the 1&#x2F;4 rule, we use it in France in FFESSM diving organisation.