King Charles stepped out in London this week to officially mark the completion of the £4.5 billion Thames Tideway Tunnel, better known as the city’s “super sewer.”

The King made his way to the ceremony via the Mars Clipper, a hybrid Thames Clipper vessel, underscoring the project’s environmental focus.

Stretching 25km beneath the River Thames, the tunnel is designed to stop raw sewage from spilling into the river, a problem that’s plagued London’s waterways for generations. Already, it’s captured over six million tonnes of storm sewage and promises to significantly boost biodiversity and water quality.

His Majesty was given a firsthand look at the tunnel’s technology, including a live tracker showing real-time data on waste diversion.

During his visit, the King also toured a planted riverwalk lined with trees and native plants, part of the project’s effort to bring green spaces and sustainability to the heart of the capital. 

He spoke with construction workers, engineers, and poet Dorothea Smartt, whose words now decorate the tunnel’s ventilation columns.

With over 25,000 people involved across 24 sites, the project — named in part after Sir Joseph Bazalgette, who revolutionised London’s original Victorian sewer system — is one of the largest infrastructure undertakings in recent UK history.

King Charles ended his visit by unveiling a plaque, officially bringing the decade-long project to a close.