"I heard after a while on the public address system that a fire had occurred inside the tunnel and the sprinkler system was going to be activated, " he told Jiji Press.

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Volunteers said they hoped the move would help with restoration of the entire tunnel and canal system.

BBC: Butterley tunnel gets Scheduled Ancient Monument status

With a wider tunnel and a stronger conveyor system running through the middle of it, one of InVision's airport machines can swallow a log.

ECONOMIST: Chopping wood

But the shut-down schedule also includes upgrades to all four of the LHC's detectors, the shielding of electronics, and even an overhaul of the ventilation system of the tunnel that houses the main accelerator ring.

BBC: LHC switches off for two-year break

Even on a dry day, the MTA's 300 pumping rooms remove an estimated 13 million gallons of water from the tunnel network, just to keep the system dry enough to run.

WSJ: Salt Water Puts City's Subway System 'in Jeopardy'

The system allows one train at a time to use the tunnel, in contrast to the several that normally use it at rush hour.

WSJ: PATH's Road Back Is Rocky

Lord Adonis, the IFG's boss and former Labour Transport Secretary, argued that London would not have had the Olympics, or Crossrail - the multi-billion rail tunnel through the heart of the city - or its congestion charge system, without an elected mayor.

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Railway historian Basil Jeuda said the tunnel was "an important link in the development of a national rail system".

BBC: Harecastle tunnel offered for sale by the government