The so-called 'Inner Tube' (Dętka) sewer runs in a circle under the Plac Wolności in Łódź, and was one of the first segments of the city's sewage system, built in 1926. It was designed by the british engineer William Heerlein Lindley (the younger Lindley) and the construction was supervised by local engineer Stefan Skrzywan. It's a large sewer, 187 cm tall and 1,5 m wide, and was used for water retention in the event that one of the connecting sewers needed purging.
They do that with machines nowadays, so the sewer was turned into a museum. It's super cool and echo-ey.