Longest tunnel under a river

The longest tunnel under a river is the Thames Tideway, a "super sewer" in London, UK. Around 19 km of its 25-km-long (11.8 mi of its 15.5-mi-long) main tunnel runs directly beneath the River Thames, following its course through the city from Hammersmith to Limehouse. The sewer's construction was completed in 27 March 2024.

The backbone of London's existing sewer system was built between 1860 and 1870 under the direction of civil engineer Joseph Bazalgette. He designed a pair of massive brick-lined sewers – called the Northern and Southern Outfalls – which intercepted existing infrastructure that drained directly into the Thames and carried it downriver for processing.

Bazalgette foresaw problems that might emerge from the city's continued growth, and so designed the system to support roughly double the volume that London (then a city of 2.5 million people) produced at the time. By the late 20th century, however, London's population had risen past even Bazalgette's margin of safety, and heavy rains would frequently cause the outfall sewers to overflow into the Thames.

The Thames Tideway is a modern "super sewer" essentially designed to do for Bazelgette's system what his system did for the sewers that had existed before. It is designed to catch overflow (typically mostly rainwater) from the Outfall sewer (as well as other unconnected systems) and transport it to a pumping station in Abbey Mills, east London. From there they can be pumped back up from the Tideway's lowest point, 66 m (216 ft 6.4 in) below the surface, and transported to Beckton Sewage Treatment Works.