Thames Water Ring Main

The Thames Water Ring Main (TWRM, formerly the London Water Ring Main) is a system of approximately 80 km (50 mi) of concrete tunnels which transfer drinking water from water treatment works in the Thames and River Lea catchments for distribution within central London. A major part of London's water supply infrastructure, the initial ring was constructed by Thames Water between 1988 and 1993 at a cost of £248 million (equivalent to £534 million in 2025), and when completed, it was the longest tunnel in the UK. Two extensions were constructed between 2007 and 2010.

Tunnel data from Wikidata (CC0). Summary from Wikipedia (CC BY-SA).