Crocodiles have been known to swallow stones, which scientists believe help them digest food and dive deeper in the lakes, rivers, and ponds they inhabit.

From the Crocodilian Capative Care FAQ:

All crocodilians eat stones and rocks in the wild – they’re referred to as ‘gastroliths’, and the process is called ‘geophagy’. These stones sit in the animal’s stomach, probably for many years, and help to break down food by a mechanical grinding action – a little like a bird’s crop. They may also help to weigh down the caiman a little more in the water and therefore act as a ballast.