Amsterdam is a city of bicycles and a city of canals. As a result, many bikes end up in the canals. Lots of other junk does, too, including shopping carts, cars, and much more.
Crane boats operate continuously to keep the canals clear for traffic. This video by Isabelle Harder shows one such crane boat at work, dredging a canal and dumping bikes and other metal objects on a barge for recycling.