We just purchased an investment property and needed to gut the basement, due to multiple unanticipated problems. Behind a wall, we found this mystery hole. It is framed with the original timbers used to build the house, so I believe it to be original. Someone has stuffed it full of debris and closed it off with a sheet of plywood and some expansion foam. The hole is perhaps a four foot cube and seems to have a dirt back wall. The roof of the hole is concrete and is likely the bottom floor of the garage. (I haven't yet been able to get everything out if the hole to verify dimensions or surface material.)

Any thoughts? What should I do with it to close it off properly? The house was built in 1962 in Ontario, Canada.

Many thanks!