The dangerous asteroid belt in the Hoth system was formed billions of years ago by the collision of two planets. Millions of boulders and rocky asteroids careen through space in orbit together, forming a deadly swarm and a menace to navigation. Entering such a field with a spacecraft would be sheer suicide.
On some asteroids there are delicate crystal ferns, which could be a primitive silicon-based life form. Over the centuries smugglers and criminals have from time to time built bases in some of the larger asteroids, including a notorious pirate who was said to have imported huge space slugs to guard his hideouts. After the Battle of Hoth , as the Millennium Falcon tries to escape pursuing Star Destroyers in the asteroid field, it is nearly swallowed by a titanic, centuries-old space slug.