Trimpley Reservoir is located near Bewdley and was built by an army of dedicated workers over four years from 1964 on former farmland next to the River Severn.
The 29-acre reservoir now provides water to parts of the county as well as south Shropshire and the Elan Valley Aqueduct in Wales, which supplies Birmingham.
Severn Trent says the reservoir is little known to people outside the local area.
Joe Upsall has been team manager of the reservoir site and the connecting water treatment works for the last nine years. He has worked some 25 years at Severn Trent, which is commemorating its own 50th anniversary this year.
The keen cyclist unveiled old images showing crews of workers digging up what was then rural farmland, laying concrete foundations, building towers and installing huge pipework to carry the water – which is some 40ft deep in places today.
Mr Upsall, aged 49, said: “They are amazing pictures from the original construction which underline what an incredible feat of engineering creating Trimpley Reservoir was.
“The project involved so many people and took so much hard work, which you can see up close in all these brilliant pictures.“
Underneath the reservoir are tunnels, described as a “James Bond-esque labyrinth of pipes below the water”.
Trimpley Reservoir is also home to its own sailing club, attracts fishermen to its banks and is a hotspot for avid birdwatchers.
During autumn and spring, gulls, waders and terns visit the site during migration and kingfishers and grey herons have also been spotted.
The surrounding Eymore Wood also shelters a variety of birdlife, including pheasants, cuckoos and woodpeckers.
Mr Upsall added: “It’s just a beautiful site, which makes it a wonderful place to work.
“Yet many people, even some living locally, still don’t know it is here because of its rural location – making it a real Hidden Treasure.”