ahahahaha.
Something similar happened up here. They where building a new studen resident building and dug the foundations ready to pour wth conrete, but after pouring the estimated volume, they needed more, so got more, then ran out, so got more, only turns out they had dug a small hole, into the hidden underground Williamson tunnels of liverpool and where basically filling, miles upon miles of huggeee tunnels. LOL
theyve been slowly excavating the tunnels, over the years and left some of the concrete in place to show the freak accident haha
https://www.williamsontunnels.co.uk/photoview.php?photo=007.jpg