Most studies on diving have been done with Alligator mississippiensis, but the general principal probably applies to most if not all species of crocodile. Studies in the 1960's showed that an American alligator could remain submerged for up to two hours. During this time, heart rate can drop to 2 or 3 beats per minute to help reduce oxygen consumption. It has also been shown that during such prolonged dives, oxygen consumption immediately after diving actually falls as the dive continues. In essence, the animal adjusts its oxygen consumption to enable it to dive for longer. Although the lungs can be used as a reserve air supply, in most cases when the crocodile dives quickly it expels the majority of air from its lungs. The crocodilian heart comes into play here, with valves closing to redirect blood to essential areas, and restrict its flow into non-essential areas - thus contributing to the reduced oxygen consumption that is seen. Crocodilian blood is also remarkable in its affinity for oxygen, carrying more in oxygen rich areas, and releasing it more quickly in oxygen deprived areas.

But there's a problem with the oft-quoted figure of two hours - under normal circumstances, crocodilians do not remain submerged for such periods. A voluntary dive normally lasts no longer than 15 minutes, and usual dive times are around 4 to 6 minutes. Studies measuring maximum dive duration before death involved holding alligators underwater - hardly a natural behaviour. Why is there such a high degree of tolerance, then? The experiments looking at dive time in alligators were concerned with resting animals. Therefore, active and stressed animals in real-life situations will be using oxygen up more quickly and reducing their maximum submergence time. A struggling crocodile underwater can actually drown in 20 to 30 minutes, as some people attempting to capture crocodiles have found to their cost. The smaller the crocodile, the lower its capacity to survive without oxygen underwater.

In times of extreme stress, extended dives can be employed by the crocodile. Such instances have been recorded in alligators where the water they are resting in starts to freeze over (e.g. near the northern limit of their range in the US) where forced submergence is caused by the surface of the pond freezing over. There is one observation of an alligator surviving submerged in near-freezing water for over 8 hours. This also illustrates the importance of temperature in crocodilian physiology - in this case, the colder the animal is the lower its metabolic and oxygen consumption rates, and so a colder crocodilian should be able to hold its breath for longer.