A team of engineers dubbed "aquanauts" are to start inspecting a massive 56-mile (90km) pipe that provides water to two million people each day.
The 80-strong inspection team have undergone training for the project to assess the Haweswater Aqueduct, a colossal engineering feat which delivers 570 million litres of water a day from Cumbria to Manchester and surrounding towns and villages.
The aqueduct was first started in the 1930s to provide fresh water from Haweswater Reservoir in Cumbria to Heaton Park reservoir in Manchester.
Much of the aqueduct was commissioned by the Manchester Corporation but is now owned by United Utilities, which has spent £22 million and 10 years planning the detailed structural analysis, with the aqueduct temporarily emptied so workers can get inside.
The inspection team have been given fitness tests and psychological training to ensure they can work in small spaces underground for long periods.
Carl Sanders, senior project manager at United Utilities, said: "The depths, confined spaces and pipe deposits will make it like another planet down there, which is why we need to be confident they are up to the task.
"Also, 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.
"It will be incredibly dark and slippery. Add to that the curvature of the pipe and you can imagine how difficult it will be to manoeuvre down there. Factor in the additional challenge of collecting structural analysis data and you can see why such detailed training is required."
To help the project, 16 specialist Vehicle Access Systems - "pipe-mobiles" - designed and built in Canada have been flown over to help transport equipment and supplies in the aqueduct.
Sarah Togher, an engineer with United Utilities, added: "It's a once in a career lifetime opportunity to be involved in a structural analysis of this scale and on a piece of engineering legacy.
"It's challenging but exciting to be involved in a project that will be a part of history in itself."
The two-week survey will help plan future maintenance work on the aqueduct to keep it flowing for years to come.