Humanite
The big underground mines deep in the Canadian Shield backfill the excavated stopes as they mine upward. They are filled with concrete mixed with all the trash from the underground work. Everything from the refuse of the miners' lunches to the used tires of the remote mucker trucks. Plus a lot of the waste rock. Buried a mile down, it will survive for a long time. Someday, in the distant future, once erosion has attacked the isostatically rising crust, these filled stopes will stand as ridges in the landscape, an anomalist breccia that any geologist would scratch their head over.