Reminds me of my last employer... they were having fiber internet pulled in to one corner of the building. Drilling company got to about the end of the run from the street and hit something metallic. Our boss says "I was afraid of that." Legend had it that an auto dealer across the alley had buried a railroad tank car under the alley to use as a fuel tank for the dealership, and yep, that legend was true!
They found the lid, hired a company to investigate environmental issues, they sopped up what was left in the tank, drilled holes in the bottom, took soil samples, all good. So they filled it in with soupy concrete. I left after the 3rd truck was empty and the tank wasn't full yet.
So someday, someone else will run into a THUMP! underground, and try and figure out why?