Well, let's see...
Dune Sandworm
Collosal Magical Beast
Hit Dice: 60d10+1020 (1350 hp)
Initiative: -2 (Dex)
Speed: 30 ft., burrow 30 ft.
AC: 35 (-8 size, -2 Dex, +45 natural)
Attacks: Bite +77 melee
Damage: Bite 10d10+25
Face/Reach: 30 ft. by 150 ft. / 30 ft
Special Attacks: Superior grab, improved swallow whole
Special Qualities: Tremorsense, immunities, regeneration 50, damage reduction 25/+5
Saves: Fort +47, Ref +28, Will +32
Abilities: Str 60, Dex 6, Con 45, Int 3, Wis 14, Cha 8
Climate/Terrain: Desert (Dune)
Organization: Solitary
Challenge Rating: 30 is a good place to start, I suppose
Treasure: None
Alignment: Always neutral
Advancement: 61+ (every 10 HD adds 5 ft by 25ft/5 ft to face/reach)
Combat:
Superior Grab (Ex): If a Dune sandworm successfully hits, it may attempt to swallow as an immediate free action.
Improved swallow whole: The Dune sandworm may attempt to swallow a grabbed opponent of any size by making a successful grapple check. The opponents size is considered one size smaller for this purpose. Swallowed opponnents take 4d10+10 crushing and 4d12+8 acid damage per round. A creature can cut his way out by dealing 100 points of damage. Once the creature escapes, muscular action closes the hole; another swallowed opponent must cut its own way out.
Tremorsense: A Dune sandworm can automatically sense the location of anything within 1000 feet that is in contact with the ground. In addition, it can tell general direction of any moving of small size or larger to a 10 mile radius
Immunities: Dune sandworms are immune to all poison and disease.