A quarry somewhere in Sussex off a disused railway line some 20 years ago when I was about 14 with my younger brother. I still have some old 35mm negatives somewhere of when we went. Machinery was still in there such as dumper trucks, cranes, a massive bulldozer and other gems, oh and about 50 million road signs, we went when Robot Wars was really popular on TV and we pinched a Road Block sign intent on making our own copycat version but we felt guilty and took it back!
My first proper explore as an adult was the Hobbit House (Colins Barn) in Gloucestershire.