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.

Boston Spa: The rise and fall of a Yorkshire spa townThe 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.

The sulphur well tunnel system beneath Crescent Gardens 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.

See behind the scenes of Grantley Hall - Yorkshire's newest luxury spa hotel"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.

Harrogate Civic Society volunteers are restoring the well-heads to raise awareness of their history

"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.

An old postcard of the Facade Wells tunnels beneath Crescent Gardens

A history of Harrogate's spa age"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."

The secret network of tunnels that runs beneath Harrogate