GAS workers dug up more than they bargained for when working on a main in Blandford.
Engineers uncovered a brick tunnel about five metres high and four metres wide, leading north-east from the junction of Edward Street and Albert Street.
Building plans nearly a century old from the town's museum were pressed into service, identifying the structure as an old railway tunnel.
Dorset County Council's senior bridge engineer, Leslie Lock, was called in to investigate the safety concerns.
"It's still active as a structure. It has highway routes running over it," said Mr Lock.
And the civil engineer highlighted the possibility of additional danger from steel drums wedged into the structure.
"We will open up the structure and remove the drums because we don't know what chemicals are inside," said Mr Lock.
Historians at Blandford Museum got the call to help flesh out the tunnel's proportions.
Plans dating back to the early 1900s were soon produced by museum staff to assist the engineer.
David Cash, a trustee of the museum, and the chairman of a railway club, talked Mr Lock through the ancient drawings.
"We have plans from 1916 which we were happy to share," said Mr Cash.
"They are the only plans we have that show the alignment of the track at the time," he added.
The tunnel led to a flour mill on the corner of Edward Street and Albert Street, said the railway expert.
"Because it was a steam powered mill, it needed coal," said Mr Cash.
"Certainly coal wagons went through the tunnel," he said, but added that he could not be certain if flour was transported from the mill by rail.
Mr Lock said once the ownership of the tunnel had been clarified with the railway company, it would be filled with expanding concrete foam.
And the engineer extended his thanks to the historians who had helped his investigations.
"There is quite a bit of history there. Any information is good information," he said.
Anyone with information about Blandford steam flour mills, especially drawings or plans, should contact Blandford Museum on 01258 450388 or the Daily Echo on 01202 886811.
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