A few years ago in Amsterdam , The Netherlands a construction company had to dig a tunnel through a former canal. The canal existed for few hundreds of years and was only a decade ago filled and a road was build on top of it. They found about 700.000 artifacts from the middle-ages till modern times. Speer arrows, swords, guns, money, credit cards, music cassettes, smoke pipes etc. I believe there is a exposition dedicated to those findings. Very interesting.