I have a huge hole in my yard. I have heard of a underground garden?

Was this were a plunge pool was? Was it man man or has it just appeared? Do you mean a sunken garden?

https://www.ehow.com/how_7661078_convert-swimming-pool-garden.html

Never heard of that but sound like a great place for a water fountain garden!

No the dirt was removed for backfill for a garage to be put in. I'm not sure it was said to be something the Indians did?

I think it was a sunken greenhouse but it did say how to set it up?

Sounds like maybe you should actually use it as a koi pool! the basic idea is line with epdm ( rubber stuff) and add some sort of water pump/ fountain to aerate the water.

Something the indians did? Maybe this was some sort of attempt to create a cave/ storage below the freezing point? A root cellar for storing vegetables? Underground caves generally have very stable - year round temps of around 55 degrees. This is intriguing! 8 or 9 feet deep IS quite a lot of material removed so I would encourage you to investigate all the possible uses so that you get some enefit of all the work so far.

I would find out what caused the hole before I did anything else. If it is a sink hole, I'd get professional help before it gets larger.

Was the garage constructed and the dirt used? I would have the hole filled in if at all possible, could become a hazard and effect your homeowner's insurance.