Scientific research up to this point, can only explain the morphing of certain species on the nation’s use of the underwater forge worker’s over use of the local fault line, tapping into heat from deep within the earth’s crust (via a fault line). I feel there is probably a more mystical force behind this.
Regardless, for one reason or another, giant tardigrades have developed, making their home in the Corrungus Forest (an underwater environment covered in giant fungii).
They’re speech is much like a whale song and is often recorded using musical notation. They seem to understand human speech well enough, (as much as any intelligent animal would) but some people have written musical scores to them. I’d imagine it sounds like a flowery speech by a non-native speaker.
The people have worked a deal with the tardigrades and they offer their services to ferry people from the underwater castle to the topside.