just out of curiousity have you searched your countys records pertaining to the property Including as odd as it sounds (if your county has one ) a historical society , to see if they have any info on how the property might have been used ? they might have info that could tell you at one point in your areas history that your property could have been part of a old landfill or a old junkyard. ( my cousins years ago had a slab home on 55th & Colfax in Brooklyn Center Mn the nieghbor across the st had lived there since the mid 20s , the neighbor told them that there was a huge landfill on that side of the street when they first moved in, it was filled over and during the 50s when the area grew many homes were built ontop of it , wich explained why they too found broken glass and more when they dug holes to plant trees) Also too some of your areas longest term residents might be able to tell you if it was something as stupid as a homeowner who didnt want any one to know how much they drank, LOL , I work with a woman who lives on the Snake river in the city limits of Pine City Mn one of the older residents on the block warned her not to permit her kids to swim out front because the previous owner back in the 60s used to smash her bottles up in the garage and then dump the glass at night into the water so no one would actually know how much she drank ) so with older property it might take some work to learn what someone else did to a place or how they used it but you might be able to find out. Good luck to you , its possible the only thing you can do is learn how to use a bobcat , or hire it out and have the first x inches of soil removed and clean fill brought in. (if you have to go that route make sure you learn what zoning will require of you )
