[Illustration: An early Napoleonic vision of the invasion of England] [Illustration: by air, sea, and a Channel tunnel.] [Illustration: Sir Edward Watkin, at the sluice-gates, vanquishes the French invaders marching on England through the tunnel. A London newspaper cartoon at the time of the great tunnel controversy.] [Illustration: THREE SOLUTIONS TO THE INVASION PROBLEM How to have a tunnel and still keep England safe from invasion is a problem that has attracted the attention of artists since the eighties. The _Illustrated London News_, 1882, shows how, at the first sign of invasion, the tunnel could be bombarded from the Admiralty Pier at Dover, from the Dover fortifications, and from positions offshore. Viaduct for the French tunnel entrance proposed in 1906. At signs of French intentions to invade, the British fleet would sail up and blow this viaduct to smithereens, thus blocking the tunnel from the French end. David Langdon in _Paris Match_, 1960, suggests another way of handling the invasion problem.] [Illustration] [Illustration] [Illustration: PROPOSED METHOD OF CONSTRUCTING A SUBMERGED TUBE UNDER THE CHANNEL] [Illustration: The illustration shows the proposed laying of a "cut and cover" prefabricated tunnel on the Channel bottom with the aid of a DeLong self-elevating construction platform.] [Illustration: Artist's impression of the boring of the double Channel tunnel, with its extra service tunnel and cross-passages, as proposed by the Channel Tunnel Study Group in 1960.] [Illustration: Three]