Choctaw.

Inuit.

First 4 letters of each word. Very simple code, add 6 to each letter.

I think we bought Alaska and gave it to the "5 civilized tribes".

Choctaw were borrowers, or at least made use of existing caverns.

Inuits built burrows in the earth at Barrow, Brevig Mission, and other villages.

Missionaries took the kids to remote boarding schools and beat the language out of them, then mixed them all around and sent them to different villages, after the elders had been killed off with polio.