To permit interchange between running tunnels, primarily to facilitate tunnel maintenance, the Channel Tunnel project incorporates two crossover caverns positioned under the Channel about a third of the way out from each portal. At the location of the UK crossover the service tunnel, normally positioned between the two running tunnels, deviates below and to the side to enable the running tunnels to come into juxtaposition over a distance of some 500 m. The central portion of this run is opened out as a single cavern measuring 165 m long, 22 m wide and 15 m high—large enough to accommodate a five-storey building and probably the largest undersea cavern built to date. The crossovers divide the length of the rail tunnels into three equal sections, any of which can be closed off in an emergency or for maintenance. At all other times, the doors are closed and the adjacent tunnels are sealed from each other. To maintain separation for ventilation purposes, the two crossover caverns are divided longitudinally by huge pairs of sliding doors. Interlocked with the signalling system, they are only opened to permit trains to switch from one running tunnel to the other. The doors are opened and closed using powerful electric motors and form an airtight fire barrier between the two tunnels. They are similar to metal doors used in bomb-proof aircraft shelters and are constructed from carbon manganese steel with a further 15mm of fire resistant material. The Channel Tunnel is no ordinary project. The four types of cross-channel service that the Tunnel offers - conventional freight and passenger trains, plus two types of road vehicle shuttle have made it into the busiest railway in the world. The fast and efficient movement of road and rail traffic into, through and out of the Eurotunnel system is integral to that success. The Channel Tunnel is one of the wonders of the modern world. It is 32 miles long and the second longest rail tunnel in the world.