An abandoned bridge in Cumbria has received national protected status, thanks to a chance visit by an amateur history group.
During a day trip to Ulverston with the Cumbria Industrial History Society, Gavin Watson, 67, spotted the "tangle of metal".
It turned out to be one of the last remaining bridges of its kind in England and has now been granted grade II listed status.
"As we walked the length of the canal... there was this bridge which was a tangle of metal and very confusing," Mr Watson said.
"Nobody quite knew what it was or how it worked.
"We spent about three quarters of an hour I suppose looking at it and scratching our heads and puzzling it out.
'Grey and anonymous'
"Then we eventually realised that it was a rolling bridge."
Mr Watson said he was only aware of one other rolling bridge in England - a contemporary one in Lincolnshire.
"This one was really quite interestingly different, and so I thought well this is a rarity."
Built in 1883 to carry The Furness Railway Company's track to Conishead Priory, the bridge was designed by local engineer Frank Stileman.
It is thought to be the only surviving example of its kind in England.
To allow boats to pass, it was designed to roll back on wheels into a small dock built into the south bank of the canal. This left a central navigable channel for boats.
A tall brick accumulator tower stands nearby. It housed a large water-filled pipe needed to operate the rams which moved the bridge.
Realising the bridge was of special architectural and historic interest, Mr Watson contacted English Heritage suggesting that the bridge be protected.
Discussions and correspondence followed, eventually leading to English Heritage advising the government that the structure be given grade II status. This was granted last month.
English Heritage said that the structure fulfilled an important place in the design and technological development of bridges.
"It [the listing] was a collaborative effort with the Cumbria Industrial Heritage Society and the other people that I spoke to. So I don't take any personal credit for that," Mr Watson said.
"I've had all those years as a civil servant - not taking credit for things, that's how I like it."
An industrial archaeologist and contributor to Pevsner architectural guides, Mr Watson is a lover of books who worked for many years writing speeches for government ministers in Whitehall.
"I have always earned my living by my pen, but my principle love has been looking at buildings and particularly old industrial buildings," he explained.
Brought up in the Whitehaven area, Mr Watson moved to the Midlands to study at university.
Later, and following a series of courses and tests spread over six months, he joined the Department of the Environment and became "grey and anonymous" for 30 years.
"The thing that I enjoyed most was re-writing the building regulations in the early 1980s," he said.
During the four-year review, 400 pages were simplified and reduced to 37 in a project that Mr Watson described as "intellectually extraordinarily difficult."
Now retired to Shropshire, he lived in London for 25 years.
Engineer John Rennie began building the 1.5 mile (2.4km) long Ulverston Canal in 1793. It opened in 1796.
The canal-side dock into which the bridge retracted has since been filled in, sealing it into a fixed position.