Leave it to the prudent Swiss to have ingenious plans for protecting themselves from nuclear war. Here's a tour of the Sonnenburg tunnel cum bomb shelter.

An excerpt:

If you are driving through Switzerland, south to Italy, you are likely to take the route via the charming town of Lucerne and that means driving through the Sonnenberg tunnel.

Those tunnels around Lucerne can be quite irritating, especially in fine weather. Just as you are enjoying a spectacular view of the lake and the mountains, you are plunged into darkness.

But when you get to the Sonnenberg, make sure your eyes adjust, and take a closer look, for this is much more than a tunnel. In here is the world's largest nuclear shelter.

Under Swiss law, local governments are required to provide shelter spaces for everyone, and in the early 1970s Lucerne was short by several thousand. The new Sonnenberg motorway tunnel, just being built, seemed a neat solution: kit it out as a nuclear shelter as well and it could hold 20,000 people.

The Sonnenberg, in theory, is able to withstand a one megaton nuclear bomb, as close as half a mile away

"Actually we got the idea from you British," explains Werner Fischer, the local civil protection chief, as he shows me around. "Londoners used the underground as shelter during the blitz."

Well maybe, but believe me, there are things in the Sonnenberg that you will never find down the Finchley Road tube station.

'Engineering feat'

It starts with the doors, which are a metre and a half thick (5ft), and weigh 350 tonnes each. The Sonnenberg, in theory, is able to withstand a one megaton nuclear bomb, as close as half a mile away.

The shelter was designed to be self-sufficient

One megaton is 70 Hiroshimas. That means the Sonnenberg residents would have emerged to a world reduced not to smoking rubble, but to ash.

Inside, the tunnel is a surreal monument to neutral Switzerland's desire to survive a total war which would, naturally, have been started and waged by someone else.

I was going to cut it off there, but there's more!

There are vast sleeping quarters, with bunk beds four layers deep. There is an operating theatre, a command post, and as Mr Fischer points out, a prison. "Just because there's a nuclear war outside doesn't mean we won't have any social problems in here," he says.

Some of my friends have private ones in their own houses, used, these days, mostly to store wine or skis.

There were even, it is rumoured, plans for a post office, until someone asked the obvious question "when the world outside is burning, who would you write to? What would the address be, not to mention who would deliver your letter?"

Then there are the coloured lights, indicating whether it is night or day outside. Obviously the country which produces the world's top watches would not like to lose track of time.