To increase capacity and to make ongoing maintenance and expansion easier, the National Grid has built an underground cable tunnel between Wimbledon and Hackney. Work commenced in 2011 and the project was operational from 2016. The tunnels are between 3 and 4 metres in diameter and have a total length of 32 km. They lie at a depth below the surface of between 20 - 60 metres. The map marker is roughly at the midpoint of the tunnels - a sub-station in St John’s Wood. The power tunnels pass beneath the River Thames just west of Wandsworth Bridge.
Transmission is at 400 kV with some sections also carrying a 132 kV supply. The total cost was around £1 billion. A second phase of power tunnels was started in 2021 and will extend the network from Wimbledon to Crayford. The length, size and cost of phase 2 is broadly equivalent to the phase 1. Tunnelling of this second phase was completed in 2023 and commissioning is planned for 2026.