It's really not. They didn't walk far enough to be out of Republic City, and this is clearly a giant sinkhole in a forest with ludicrously enormous trees.

"What are you?" Beila asks the spirit.

"The pocket," says the spirit.

"A pocket of - something under the ground? Magma, water, air -"

"Yes."

Beila gulps.

"Who promised you?"

"Humans," says the voice. "Lying humans."

"Names. A company, an individual, a group -"

"Humans," repeats the spirit.