shallows, with no apparent ill effects from the lack of basking on land. In South Africa, Nile crocodiles are more easily observed in winter because of the extensive amount of time they spend basking at this time of year. More time is spent in water on overcast, rainy, or misty days. In the southern reaches of their range, as a response to dry, cool conditions that they cannot survive externally, crocodiles may dig and take refuge in tunnels and engage in aestivation. Pooley found in Royal Natal National Park that during aestivation, young crocodiles of total length would dig tunnels around in depth for most, with some tunnels measuring more than , the longest there being . Crocodiles in aestivation are lethargic, entering a state similar to animals that hibernate. Only the largest individuals engaging in aestivation leave the burrow to sun on the warmest days; otherwise, these crocodiles rarely left their burrows. Aestivation has been recorded from May to August. Nile crocodiles usually dive for only a few minutes at a time, but can swim under water up to 30 minutes if threatened. If they remain fully inactive, they can hold their breath for up to 2 hours (which, as aforementioned, is due to the high levels of lactic acid in their blood). They have a rich vocal range and good hearing. Nile crocodiles normally crawl along on their bellies, but they can also "high walk" with their trunks raised above the ground. Smaller specimens can gallop, and even larger individuals are capable of occasional, surprising bursts of speed, briefly reaching up to . They can swim much faster, moving their bodies and tails in a sinuous fashion, and they can sustain this form of movement much longer than on land, with a maximum known swimming speed of , more than three times faster than any human. Nile crocodiles have been widely known to have gastroliths in their stomachs, which are stones swallowed by animals for various purposes. Although this is clearly a deliberate behaviour for the species, the purpose is not definitively known. Gastroliths are not present in hatchlings, but increase quickly in presence within most crocodiles examined at and yet normally become extremely rare again in very large specimens, meaning that some animals may eventually expel them. However, large specimens can have a large number of gastroliths. One crocodile measuring and weighing had of stones in its