ESTIMATEDDate confidence: ESTIMATEDA target, not a fixed dateHow firmly the source words the date: confirmed, scheduled, estimated, delayed or unverified.Show all ESTIMATED pins65% CONFIDENCEOverall confidence: 65%How well the pin's source and references back up its dates.Weighted average of how firmly 3 references, the source included, support the pin's start and end times; a reference counts half as much for every 180 days older than the newestShow all pins from 50% to 74% confidence
The Paris[3] sewer museum's own history page states tours "began in 1867, during the World's Fair" without naming an exact day, so the pin uses the Fair's opening date.During the 1867 Exposition Universelle, Paris opened Baron Haussmann's newly rebuilt sewer network to public tours for the first time, turning Eugène Belgrand's engineering into a civic showcase.
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