Nile crocodiles usually dive for only a few minutes at a time, but can swim under water up to 30 minutes if threatened, and 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 on occasion of surprising bursts of speed, briefly reaching up to . They can swim much faster by 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 , more than three times faster than any human.