Perry Mandeville and the other Eton boys had been playing football when they heard the bang. They were out in the fields next to Ugo’s stadium and they stood now, shielding their eyes from the glare and straining to see what was happening.

Peter Haight and Mr Cooper-ffrench had disappeared and Quintino, their Italian guide, had been supervising them. Left to their own devices, they had done what boys everywhere will do and started kicking a ball around. All morning they had heard the strange noises from up at the palazzo and assumed it was part of the carnival, but this latest mighty thump had sounded different.

‘I wonder what’s going on up there?’ said Tony Fitzpaine, his heavily bandaged nose making him sound stuffy and nasal.

‘Oh, they’re m-mad, the Italians,’ said Perry. ‘Quite m-mad…’

James was still holding his breath. He felt an unbearable tension. Zoltan was standing staring at the lake, his right fist clenched.

‘Come on,’ he whispered. ‘Come on, damn you.’

Then James saw the gentlest puff of dust rise from the ground; next to it a second puff danced into the air, and then a third plume that zipped along the rock, like the dust thrown up behind a speeding animal, as a long black fissure opened in the top of the ledge.

‘Oh, my God,’ whispered Zoltan. ‘She is going.’

He was right. The whole ledge began to topple over, slowly at first, then in a great rush, as half the mountainside came away.

It was a terrifying sight, 100,000 tons of rock crashing downward in one great crumbling, roaring mass.

As it slammed into the lake there was a deafening crash and water was sent steaming up into the sky, so high it blotted out the sun for a moment. One minute it had been a calm, sunny afternoon and the next James was in the centre of a torrential downpour of stinging water so thick he couldn’t see his hand in front of his face. When the air cleared he saw something he could hardly believe: a vast, 20-foot-high wave was travelling along the length of the lake.

The surging water scraped the edges of the basin, tearing up trees by their roots and wiping out the little run of buildings on the other side. Then it picked up the big white seaplane like a child’s toy and tossed it over the side of the dam.

James watched as the plane glided a little way out across the valley before tipping and plummeting downward.

Ugo was standing on the piazza, transfixed, one arm raised to give the signal to his men, looking up, unable to comprehend what was happening, as his beloved Sikorsky flew over the dam and dropped towards him.

Behind the plane a solid block of water filled the sky.

‘No!’ he said and stretched his hand out still further as if to hold it back. ‘No!’