There have been questions about sizing giant animals, giant humanoids, giant flying creatures (also discussed particularly for dragons and giant floating mammals), and giant spiders, one type of invertebrate. The first link has some general applicability here, but I am particularly interested in the mechanics and feasibility of a different type of invertebrate, giant worms, similar to what this question mentions:
60 feet in length and 20 feet in girth burrow up from under the mountain
That question has the worms carnivorous, but I am more interested in a generally docile worm, much like a large earthworm (though obviously at that type of size, could incidentally seriously injure or kill a person).
Parameters for Feasibility:
- Environment: Lives on Earth-like world (gravity, atmosphere, etc.); essentially, if in question, pretend we are talking about Earth.
- Ecosystem: Land based (burrows like an earthworm, though [preferably] may be considered to in some way penetrate harder rock than just loose earth); may be able to swim as well; must dwell exclusively subterranean with possible deep ocean excursions (whether swimming or just poking itself out of the earth at deep ocean depths).
Size: 10-15 feet in diameter at the most, but may be however long it is deemed necessary within reason; proportions of earthworms do not directly scale width/length according to the previous Wikipedia link that states:
10 mm (0.39 in) long and 1 mm (0.039 in) wide to 3 m (9.8 ft) long and over 25 mm (0.98 in) wide [the larger size being the Giant Gippsland earthworm]
I would not desire my 10' diameter worm to be 1000' long! So they will be a wider, stouter variety (more like a Goliath Beetle's larva proportions, which "are capable of growing up to 250 millimetres (9.8 in) in length" with a much thicker shape). Something in the 50'-150' range seems reasonable to consider.
- Shape: May have some type of small "leg-like" appendages either in the front or full length, but generally still a worm more than a centipede, so either very few, short stout legs (like the grub example above) or smaller, almost "hairlike" (in proportion to its size, anyway) legs the run the length.
- Diet: Not naturally carnivorous, though if they are "capable" of eating flesh, that is okay; ideally, while earthworms subsist not off earth itself, but plant matter in the earth, some consideration of how they might actually digest and subsist off earth/rock would be nice (acidic juices that "dissolve" the rock before ingestion, etc.), since they are purely subterranean.
- Intelligence: Nothing more than animal (or even that of an earthworm) needed.
What things need to be considered to have such a worm be "naturally" feasible? A small suspension of belief is acceptable, but generally what kind of systems (physiological, ecological, etc.) would it need to have in place to grow to that size and possibly live off "earth" (or if not, what would it need to survive)?