The Thames Tideway under the River Thames is the biggest overhaul of the London sewage system since the Victorians built the first one.

It runs 25 kilometres (15.5 miles) long, is over seven metres (23 feet) wide, and reaches a depth in some places of 67 metres (220 feet).

Before the Tideway was built, 40 million tonnes of sewage flooded into the Thames each year.

It won't replace the old tunnels, but will ensure that whatever escapes will be caught and flow to water treatment works.

The company which owns the Tideway, Bazalgette Tunnel Limited, is named after civil engineer Sir Joseph Bazalgette, who built the original tunnels.

CEO Andy Mitchell says the Thames Tideway will fundamentally change the environment of the river from potentially hazardous to healthy.

The hope is it can meet future needs of the capital's population of nearly 9 million people, according to the Trust for London.

Mitchell believes that the Tideway may easily exceed expectations, removing as much as 95 or 98% of the hazardous pollution.

But he warns that if the weather continues to get wetter and flooding becomes more frequent, the tunnel alone will not be enough to catch everything.

He explains that London, like many other UK cities, has been buried under concrete, and there are just not enough green areas to soak up the rain.

But Mitchell is confident that the first water tests they carry out will show a big improvement in quality.

This in turn will hopefully increase the amount of wildlife and diversity in the Thames.