I've just turned 90. And I'm going to water-bike 104 miles on the Thames.

I know. I know.

But hear me out, because this isn't just about a slightly mad old man on a peculiar contraption. This is about something I've spent a lifetime caring about and am not willing to give up on: Our Rainforests.

The Challenge: Oxford to London by International Rainforest Day

This summer, I'll be climbing onto a water-bike in Oxford and pedalling all the way to London. That's roughly 104 miles. Four days. 26 miles a day. A marathon each day, on water with at least 31 locks to navigate along the way, finishing on Monday, 22nd June: International Rainforest Day. I can think of no better day to end this challenge.

I have a water-bike at home in Cornwall. I'll be training on my pond and an exercise bike. At 90, you work with what you've got, and you keep going.

Each morning, a well-known face will join me on the water for a stretch, and we'll be talking to local and national media along the route. There'll be my family, an escort boat and a small film crew with me throughout. It should be quite an adventure.

Starting at Magdalen Bridge, Oxford on Friday 19th June, I will set off early in the morning, pedalling through to Teddington over four days, finishing on Monday 22nd June (International Rainforest Day).

Why I'm doing this

Most people don't know that Britain has rainforests. It does. Or rather, it did. Ancient, moss-draped, extraordinarily biodiverse temperate rainforests that once clung to our Atlantic coastlines. I've spent decades exploring the great rainforests of the world, and what many don't realise is that one of the most remarkable was right here, on our own doorstep.

Now it's almost gone.

I've seen what we've lost. I've watched forests disappear across my lifetime. And I've reached an age where I feel, rather urgently, that if I'm going to do something about it, I'd better get on with it.

That's why I'm asking for your help.

The money raised will go to the Thousand Year Trust, a charity that does extraordinary work researching and protecting Britain's temperate rainforest and is carried forward with remarkable dedication by my son, Merlin. It means a great deal to me that he has taken up this cause and made it his own.

With your support, we're aiming to raise £150,000 to help fund something that has never existed before: Europe's first temperate rainforest research station.

A permanent home for the science, the conservation, and the ambition that Britain's rainforest deserves. Something built to last, not just for us, but for the generations who come after.

A once-in-a-generation effort. And I mean that quite literally, I'm 90 years old, and I've never meant anything more sincerely in my life.

Every donation helps bring the station closer to reality. Here's what yours could do:

£25 supports our native tree nursery, helping replant the very species that will one day surround the research station.

£100 will fund lighting designed to protect the Dark Skies status of Bodmin Moor and the nocturnal wildlife that depends on it.

£1000 will help fund the equipment within the research station for scientists and students to carry out this cutting edge work.

£5,000 will help building the community hub at the heart of the station, the kitchen, workshop and gathering space where families, researchers, conservationists and the local farming community will come together.

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Please sponsor me

I'm not asking you to get on a water-bike. I'm just asking you to believe, as I do, that it's not too late, and that what we build now can protect something irreplaceable.

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Every donation, however large or small, gets me one pedal stroke closer.

Thank you, sincerely.

Robin Hanbury-Tenison OBE.

-- Explorer, Conservationist, Author. 90 years old and still at it.