On this dive, Whittaker and Craig each carried five air tanks. Whittaker, a newcomer to the wreck, breathed normal air, which is 21 percent oxygen and 79 percent nitrogen. More and more scuba enthusiasts are using trimix, though it was once used exclusively by military and commercial divers. Trimix has pros and cons. It helps dampen narcosis.
Divers overestimate their abilities, going too deeply into a wreck, for example.
So sometimes, people make bad decisions.