The river London decided to build under

Summer on the tidal Thames used to come with a catch. When heavy rain filled London's Victorian sewers, the overflow went into the river: up to 39 million tonnes of sewage in a year.

Chris Weston, who runs Thames Water, gave the reason in one line: "London's Victorian sewer system was built for four million people but today, it serves over nine million."

So the city built a second sewer beneath the old one. The Thames Tideway Tunnel runs 25 kilometers under the river and made the last of its 21 connections in February 2025. By then it had already stopped 5.5 million cubic meters of sewage from reaching the Thames. With it connected, storm sewage spills into the tidal river are expected to fall by about 95 percent.

This is prevention at the scale of a capital city. It does not clean the river after a storm. It keeps the worst of the storm from arriving, which is the part of the work a river notices first.

Rivers keep score slowly. The fish, the birds and the people on the foreshore will tell the rest of this story over the next decade. The early sign is simpler: a city looked at its river and decided it was worth building under.

Tideway on the final connection, February 2025: https://www.tideway.london/news/press-releases/2025/february/london-s-super-sewer-now-fully-connected-promising-a-greener-healthier-river-thames/ The Thames Rivers Trust briefing on the tunnel: https://www.thamesriverstrust.org.uk/about-thames-rivers-trust/briefings/briefings-the-thames-tideway-tunnel/