They're the subterranean relics of Harrogate's heyday as a genteel spa town.
Police officers conducting security checks in the town centre ahead of the UCI World Championships cycling event had to descend underground to scan a network of fascinating Victorian tunnels for suspicious devices.
Their presence shone a light on a forgotten part of Harrogate's history, and one which played a major role in the town's prosperity.
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The tunnels are the remains of the sulphur well system, which once pumped mineral-rich waters from the springs to the numerous spa baths. There are also a series of underground reservoirs that still contain water today.
Local archivist and retired librarian Malcolm Neesam is an expert on the sulphur wells and has included segments on their function in his new book about the history of the spas, Wells and Swells, which will be published later this year.
"Harrogate is riddled with these tunnels. They date from 1834-97, when most of the baths were built. The wells often went down quite a depth to ensure the surrounding soil didn't pollute the shafts.
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"There were inspection tunnels for engineers to monitor the reservoirs, which were in big brick chambers."
Amazingly, nobody knows exactly how many tunnels are still beneath landmarks such as Valley Gardens and the Royal Baths.
"The maps and plans didn't survive - in the 1960s, once the spas were dead, they were thrown away. There is no complete survey of them still available."
Harrogate Council are responsible for their maintenance, and their staff occasionally go down below to conduct safety checks. Malcolm himself has been in the tunnels, but thinks it is unlikely there will ever be public access to them.
"You have to crouch to get through them and are bent double in most of them. There are these vast brick vaults - it's a bit like Phantom of the Opera! The reservoirs haven't all been drained.
"The tunnels fell out of use in about 1969, when the last spa closed, and were forgotten about by the 1980s. The entrances were sealed with lids, but if you know where to look you can still see the metal plates.
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"Volunteers from the Harrogate Civic Society have recently been painting some of the entrances to raise a bit of awareness that they're there.
"I think it's unlikely there will ever be tours of the tunnels - they are very steep, and they're not meant for people other than the engineers."