Simply because, and not wishing to become to pedantic, the tunnel is filled (slightly pressurised) with good old fashioned air as it is a tunnel about 75m (250 ft) deep under the sea-bed at its lowest point, and 115 m (380 ft) below sea level.
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The Channel Tunnel is open at both ends, so the air pressure inside the tunnel is pretty much the same as the air pressure at sea level (just a little more, since the channel tunnel is at a lower altitude than sea level).
The tunnel is actually designed to leak. As Eurotunnel explain on their website, 'seawater from the rocks above the tunnel drips through and is then pumped away.
How fast does the Eurostar go? The Eurostar travels through the Channel Tunnel at a speed of 100 miles per hour (160kph) although when the train is outside the tunnel it reaches speed of 186 miles per hour (300 kph).
1 Answer. To allow for adequate fire suppression, in the event of a vehicle fire in the tunnel. The tunnel's fire suppression system depends upon complete mixing of the released halon gas with all air, without having pockets of high (or low) halon concentration.
The Eurostar runs at ground level so most people don't suffer from any issues at all. However, in particular when crossing the Channel Tunnel, there is a slight change in pressure and I often get “closed” ears that I have to pop. I have heard of a few people suffering from headaches when crossing the Channel Tunnel.
This accident of geology was one of the two reasons why the fixed link is a tunnel and not a bridge. The other reason is that the Channel is the busiest seaway in the world, with over 600 shipping movements each day. Any bridge or other structure in the Channel would almost certainly be rammed by a ship in due course.
The Laerdal Tunnel in western Norway is the longest road tunnel in the world. It takes about 20 minutes to pass through the tunnel. Brightly colored lights placed every six kilometers help drivers stay alert.
The cars board specially crafted train cars, then the train leaves, enters the tunnel and exits on the other side, where cars roll off. It is a short passage: 35 minutes only. It is not more claustrophobic than taking a metro ride. But of course, if you suffer from claustrophobia, it may be a difficult experience.
On 11 September 2008, a France-bound Eurotunnel Shuttle train carrying heavy goods vehicles (HGVs) and their drivers caught fire while travelling through the Channel Tunnel. The fire lasted for sixteen hours and reached temperatures of up to 1,000 °C (1,830 °F).