Leviathans are the giant creatures that make Subnautica genuinely scary. Most you avoid, a couple you ignore, and one runs the whole story. Here's every Leviathan-class creature, exactly where it lives, how many there are, and how to handle it.
25 total: 9 in the Crash Zone (one guards the Aurora), 8 in the Dunes, 7 in the Mountains, 1 at the edge of the Bulb Zone. Hunts by engine noise — run your Seamoth/Cyclops silent nearby. A Stasis Rifle only holds it if you hit the head. Avoid; don't try to fight it.
Grand Reef (2), Northern Blood Kelp Zone (1), and the Crater Edge (the void), where they respawn endlessly — effectively the map's "do not swim out here" wall. Extreme threat; avoid the void entirely.
Exactly 3, all in the Lost River (Bone Fields, Ghost Forest, Mountains Corridor). Smaller and weaker than the adult, but still deadly on foot — stay in a vehicle.
3 total: 2 in the Inactive Lava Zone, 1 in the Lava Lakes (the deep endgame volcanic region). It throws fireballs and a direct bite is an instant kill. You navigate around it on the way to the Primary Containment Facility — it's a hazard, not a scripted boss fight.
Drifts slowly across nine biomes (Grassy Plateaus, Mushroom Forest, Bulb Zone, Blood Kelp Zone, Crag Field, Dunes, Grand Reef, Sparse Reef, Underwater Islands). Never attacks — but Tiger Plants often grow on its shell and shoot, so don't hug the back. You can harvest barnacles on it for Copper/Silver.
Walks the seafloor in small herds along the Sea Treader's Path (also Grand Reef). Harmless — and useful: it stomps Shale Outcrops (rare materials) and its leavings yield biofuel. Note it's immune to the Stasis Rifle.
The largest creature in the game (~160–200 m) and entirely passive — you can't attack it and can't even scan it. There's exactly one, held in the Primary Containment Facility aquarium (~1,200 m down).
It's central to the plot: it communicates with you telepathically, and curing the Kharaa bacterium (Enzyme 42) means hatching its eggs. A narrative beat, never a combat encounter.
Not as a living creature in the base game. What exists is the Gargantuan Fossil — an extinct ~3-million-year-old skeleton (an eel-like super-predator, estimated over 1,000 m) in the Bone Fields of the Lost River. It's scenery and lore, not something that can hurt you. A living Gargantuan only appears in a fan mod, not the official game.
Survived the deep? Here's the rest of the crater.