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Brussels/Belgium is full of them.

My favorites are the Clockarium, Museum of the Art Deco Ceramic Clock. (https://www.clockarium.org/ Check out the video, its is magnificent.)

And The Sewer Museum: Experience an authentic sewer, stroll along the Senne and discover the little-known but ever so important profession of a sewage worker. Descend deep into the bowels of the city for this unique experience! https://sewermuseum.brussels/