Chambers within backyard

Hi, I had trouble finding an answer, or finding a proper key word to search, to an observation.

Our cat recently died so I decided to dig up part of the backyard and what I found was a hole, or chamber, half a metre down. The size was about 24" by 24" and 12" depth. I found another one close-by but alot smaller.

My thoughts are that there maybe many 'chambers' around the backyard. My question is what is it and how is it formed?

I'm guessing a tree may of been removed, with the roots removed throught where the trees' base was. When they fill the hole , it didn't fill the holes that the roots came from.