They are enormous animals, growing up to hundreds of meters in length, that burrow beneath the sands of the desert planet Arrakis. They begin their lives as sandtrout before metamorphosing to sandworms. They have very long lifespans, in the thousands of years, but are vulnerable to water, which is poisonous to sandworms though not to sandtrout.
They produce, as excretions from their body, three important substances: oxygen, the sands of Dune, and the extremely valuable spice melange.
They are very dangerous, as they will aggressively attack anything that produces vibrations on the surface of the sands. They are, however, ridden using "maker hooks" by the Fremen, who revere or even worship them.