Can Aniimo burrow underground?

Burrowing is one entry in a longer list. Every Aniimo brings its own physical traversal ability while twined, and across the roster that covers sprinting, climbing vertical walls, swimming, diving, gliding, flying and tunnelling. So the ground opening up for you is a team composition question, not a progression unlock. Bring an Aniimo with the burrowing trait or the ground stays solid.

The point of the tunnel is where you come back up. You go under, re-emerge, and interact with something on the field that you could not reach on foot. That puts burrowing in the same family as the other ability-gated interactions in the world: certain Aniimo forms and abilities act directly on world objects and solve environmental puzzles, and a blocked path is usually a question of which creature you have with you rather than how strong it is. Ice obstacles, for instance, need a fire element Aniimo's abilities to melt. A burrower will not help you there. Worth knowing too that some twining abilities overlap with what you can already do, since your character can swim across water unaided. Nothing on foot digs.

Getting hold of a burrower is its own problem Which wild Aniimo appear in the world shifts with the in-game day-night cycle, the weather and other natural phenomena, with weather in particular driving when unique Aniimo show up. That means the creature you want may simply not be out when you go looking. Completing regional exploration activities unlocks map-wide perks including area and weather control, which is the lever that turns that from luck into planning. Until you have it, catching a specific traversal specialist means going out when the world is in the right state.

Not settled: The burrowing trait sits on specific Aniimo rather than on your character, so it leaves your kit the moment that creature leaves your team. Which species carry it is still shifting between builds.