This is a story of an ancient island, an extinct giant, and a mystery that I've been puzzling over for half my life.

Fifty years ago, I came here to the island of Madagascar to make a series of programs about the island's remarkable wildlife.

That was way back in the early days of television when everything was in black and white.

It was one of the first natural history series that I'd made.

Madagascar lies in the Indian Ocean, here, and even on a globe this size, it looks a tiny island, perhaps because it's dwarfed by this vast continent of Africa.

But in fact, it's an immense island, over a thousand miles long, bigger than the British Isles.

I was astonished by the animals I saw.

They were unlike anything living elsewhere.

And while I was here, much to my surprise, I acquired an extraordinary object that has been one of my most treasured possessions ever since.

Down in the south of the island, I found lying in the desert sand, pieces of what looked like very thick eggshell.