Mornin'!

Looking for some help to discern what I am dealing with. Not looking for specific models or anything, just hoping to get information to better hone my searches to see what sort of volume or hazards I may be dealing with.

I have noticed a bit of a sinkhole developing in my back yard in the spring--the volume so far is approximately the size of a barrel (100gal?). Sink hole appears to be developing above some sort of sheet metal container (size unknown). I have not yet excavated this, but have opened up around sinkhole to hopefully better see what I'm dealing with.

Some notes on the steel container:  
\-Possibly cylindrical

\-Variably collapsed(?)

\-Appears to have folded edges/seams

\-Buried just shy of 4' deep.

\-Spotty rust

\-2 1.5" black PVC pipes appear to have been connected at one point (when discovered, appear to be disconnected), one \~3 ft deep and the other \~2ft deep. These do not have labels printed on them and are somewhat flexible.

\-Thin sheet metal not cast iron

\-Variable hollow thud when tapped, sounds sort of like plastic

\-Does not appear to have any cement external supports/jerseys

Notes on where it is buried:  
\-Property in northern Ontario, Canada, 1960's era

\-50-60ft from back of house, down slope

\-\~15-20ft away, down slope from former in-ground pool (which is backfilled), pool likely 80's or 90's

\-possible irrigation or bird-bath or back yard cement ponds on the property at one point

I have been looking into septic tanks or dry wells, but not certain that is what I could be dealing with. I simply want to backfill.

Thank you in advance for tips/advice/knowledge!

What is this buried metal tank with PVC piping?