Flood defence contractors have discovered a vast underground chamber, thought to be engineered by Isambard Kingdom Brunel.
Work on the £4M flood defence scheme to improve protection in Starcross and Cockwood in Devon has been put on hold while archaeologists assess the chambers.
Environment Agency contractors were carrying out work to raise a car park in Starcross by around 20cm to reduce flood risk from a nearby slipway when they discovered a “very old manhole”.
The manhole led to two underground chambers, thought to be linked to a nearby Grade I-listed, 19th century pumping house which powered Brunel’s failed atmospheric railway.
Under the atmospheric railway system, vacuum traction was used to move trains along the tracks. Steam-driven pumping stations along the route extracted air from pipes running along the middle of the tracks, which propelled the trains forward.
The chambers are thought to have held the water which powered the steam-driven pumping station at Starcross.
Environment Agency project manager Rob Butler told that New Civil Engineer nobody had known about the 30m long, 3m wide chambers as they were not included on historic maps.
He said: “There is a note on the listing for the structure that talks about underground chambers, but we were under the impression that they were actually underneath the structure of the nearby pump house. So they might be completely different, we don’t really know.”
Butler added it was “very unlikely” the structures are separate.
Although the chambers have not yet been declared safe for humans, laser scanning has been carried out to get their size and shape.
Butler said: “At the minute there’s a dumpy bag over this hole so no-one can get down it, and that’s about it.”
Brunel’s atmospheric railway was ambitious but ultimately unsuccessful, and was shut down in 1848 after less than a year due to spiralling costs.
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