Islington Tunnel
The 451st longest tunnel in the world
The Islington Tunnel takes the Regent's Canal 960 yards (878 m) under Islington, as the longest such tunnel in London. The way for short boats and barges only opened in 1818; the pavements above are waymarked so the otherwise discontinued towpaths are connected. The canal's Eyre's and Maida Hill Tunnels, to the west, are much shorter.
Tunnel data from Wikidata (CC0). Summary from Wikipedia (CC BY-SA).