A 19th Century engineering project by Isambard Kingdom Brunel has scuppered a long-awaited road widening scheme.
Funding for the work in Starcoss, Devon, was allocated last month, but work has been suspended with the discovery of a "nationally significant" underground reservoir.
It was part of Brunel's atmospheric railway, which used a vacuum system to move trains.
Structural assessments are under way and archaeologists are being consulted.
Although considered a genius with his work on the Clifton suspension bridge, Great Western Railway and SS Great Britain, the Victorian engineer's atmospheric railway failed.
It moved the trains by extracting air from pipes laid between the rails, using pumping stations along the track.
The pumping stations used water from the underground reservoir to create steam.
The system proved more expensive than using conventional steam-power and operated for less than a year.