Tunnels (as mentioned below), sewage systems, foundations of large structures, detritus, glass and ceramics. Most modern concretes would decay quite rapidly on

Tunnels (as mentioned below), sewage systems, foundations of large structures, detritus, glass and ceramics. Most modern concretes would decay quite rapidly on these scales, metals would disperse, plastics get eaten. The patterns we have made on multi-kilometer scales will remain for very long timescales. We have altered floodplains, water tables, and ecosystems so much so quickly that we will recognise artificial desertification for a very long time indeed. And we'll still be confused about whatever the fuck we were up to, other than enjoying the human condition :)