Back in high school (late 70s), I was walking in a wooded field about a half mile from school, and just across the street from a VA hospital.
My friend and I cam across a crater, about 2 feet deep and 4 feet around.
Around the crater was a lot of shattered green glass bottles. I looked at a label connected to some shards and it was some sort of chemical - I don’t remember which specifically. But I do remember the label had a warning: “Explodes rapidly when heated.”
So it was a medical waste dump from who knows when which self exploded. I think it may have been there for quite a few years as the field was overgrown.
We used full and half-sticks of dynamite, depending upon stump size, back in the 50s & 60s, to blow stumps in clearing fields, and they’d leave quite a crater, with the stumps and root systems being 15-20ft away from the crater.