An underwater camera and doorbell at the Weerdsluis lock on the Oudegracht, installed so that the public can help fish migrate upstream. In spring, fish gather at the lock waiting for it to open, but the lock keeper can't always see them. The solution: a livestream camera shows the underwater view, and anyone — anywhere in the world — can press an online button to ring the doorbell, alerting the lock keeper to open the gates.
It is gloriously, absurdly Dutch: a practical solution to an ecological problem, implemented with total seriousness and a hint of humour. The camera feed is available online during the spring migration season (roughly March to May). You can also visit the lock itself, on the northern edge of the old centre, and watch the fish in person. Free, always accessible, and guaranteed to make you smile.