I am going to take a longshot guess here. Backhome, before garages and service stations accepted used motor oil, some people buried steel tanks in the ground with a standpipe to store the used oil and not pour it out on the ground to contaminate groundwater. I have seen a couple over the years and they looked very similar. Since it is on your property and you are responsible, you may want to cut the lock off, remove the cap and use something to stick down in there and get a sample. Hopefully it is not leaking into the ground from a rusted out tank and you will be responsible for the cleanup.