A specialist team of 'aquanauts' is preparing to venture into one of the UK's most important water pipes.
United Utilities is undertaking a detailed structural analysis of Haweswater Aqueduct in Cumbria's Lake District for the first time.
The team of about 100 men and women face long hours in tight, cramped conditions.
They are undergoing specialist training in Kendal on how to move safely in difficult surroundings
Engineers must wear sterilized suits in the tunnels which supply drinking water to 1.5 million homes across South Cumbria, Central Lancashire and Greater Manchester.
Mick Flounders, of Hartlepool, is one of the men selected to go into the tunnels.
"You're not aware of what's above you. You're encased, but it's normal for us.
"The first time you go down it's overwhelming, but once you get down there you just get on with your job."
Carl Sanders, a senior project manager with the company, added: "Tunnels are, by their nature, dark and confined.
"Some are just under three metres in diameter, but they're also slippery. I want to get all my men and women in and out safely so training is key."
To prepare them fully, a simulation facility has been constructed in Kendal.
Mr Sanders added: "With some sections of pipe 19km long and hundreds of feet below ground level, ensuring the safety and wellbeing - both mental and physical - of our crews that enter the aqueduct is vital.
"The depths, confined spaces and pipe deposits will make it like another planet down there.
"Each crew member will need to pass strict tests to show they can cope with long periods in small spaces."
Commissioned in 1955 by the Manchester Corporation, the aqueduct was a huge engineering feat. Today, 570 million litres of water flow from Haweswater to Manchester daily.