The sandworm was a native life-form of the planet Arrakis. It lived in the vast deserts and sand dunes that stretched across the surface of the planet. Most importantly, sandworms are an essential factor in the creation of the Spice Melange.
Sandworms lived beneath the sand. Attracted to rhythmic vibrations on the surface, they would breach in pursuit of the origin of such vibrations. This was an effort to defend their territory, of which they were highly protective. Thus to see a worm, and live to tell about it, was extremely rare