Thames Tideway Tunnel
The Thames Tideway Tunnel is a deep-level sewer along the tidal section of the River Thames running 25 km (16 miles) from Acton in the west to Abbey Mills in the east, where it joins the Lee Tunnel which connects to Beckton Sewage Treatment Works. The tunnel is designed to capture almost all the raw sewage and rainwater from combined sewers which would otherwise overflow into the river during heavy rain. The sewage can be stored in the tunnel until it can be treated at Beckton.
Construction of the Tideway Tunnel began in 2016.
The main tunnel has an internal diameter of 7.2 m (24 ft) and runs at a depth of between 30 m (98 ft) at the western end, and 70 m (230 ft) in the east. The tunnel will drain 34 of the most polluting combined sewer overflows (CSOs).
The above information is takne from the detailed Wikipedia entry.