Woodwalton Fen is one of Britain's oldest nature reserves - established in 1910. A 500 acre rectangle of water, woodland, scrub and marsh, it sits amid some of the richest and most intensively-cultivated farmland in Britain, the Cambridgeshire Fens.
Woodwalton is home to a number of species which once flourished in the surrounding landscape, but which have now largely vanished.
It's a wonderfully quiet, wetland oasis where waterfowl, occasional water voles and plants like ragged robin, greater spearwort (a kind of buttercup) and tall marsh sow thistle still survive.
Among them are one or two real rarities like the tiny and delicate fen violet, which flowers at this time of year and which is found in only three places in Britain.
As the manager, Alan Bowley, explained to me Woodwalton is a fragile place. It's relatively small - and with 400 species all competing for space and survival it's difficult to manage; there's always the danger that more vigorous species will eliminate less robust competitors.
And perversely for such a damp place, it's also at risk from flooding: the wetland habitat is maintained by a tall bank, or bund, running right round the reserve. Inside, the ground level is actually some three feet higher than the surrounding fields.
But if winter flooding threatens the farmers' fields, Woodwalton serves as a flood reserve, into which huge amounts of excess water can be pumped.
Unhappily, as well as simply drowning plants, the floodwater is full of fertiliser, sewage and other nutrients: far too rich a mix for some of Woodwalton's rarities to handle.
The intensively-farmed fenland landscape is the product of centuries of human intervention, as the rich peat built up over thousands of years was drained to produce a wonderful soil ideally suited to crops like potatoes.
Holme Fen Post now stands about 4m (15ft) above ground
Just a few miles from Woodwalton is a second nature reserve, Holme Fen - now largely given over to silver birch - which boasts a famous landmark.
When the Victorians began to drain a nearby mere, the local landowner, William Wells, decided to sink a cast-iron post into the peat to see how much it shrank as it dried out. The post's top was at ground level in 1848; its base rested on the underlying clay.
Today, the top of the post is an astonishing 4m (15ft) above the ground - and Holme Fen is the lowest point in Britain, many feet below sea level.
To safeguard Woodwalton and to recreate the old fenland habitat on a much larger scale Natural England (which runs Woodwalton) and partner organisations including the Wildlife Trust came up with an extraordinary - and ambitious - plan at a cost of more than £15 million.
The Great Fen Project aims to buy up all the farms between the two nature reserves and immediately around them; take them out of intensive arable production; allow the water back in; and encourage them to revert to something close to their original marshy state.
In time, the new reserve will cover a massive 9,000 acres - and who knows, the peat may even start to regrow.
The process has already started. Darlow's Farm, immediately to the north-east of Woodwalton, was bought in 2004. The tractors have vanished, to be replaced by pools of standing water in what were once potato fields, and by cattle grazing the fenland meadow.
In December, farmer Jonathan Papworth sold his 450 acres next door to Woodwalton and Darlow's to the Great Fen Project. Over the next three years his fields too will be taken out of production.
And this week the project announced the purchase of another 3,200 acres from the Crown Estate.
It's an impressive achievement. But one question troubles me when I visit. Should such rich arable land be taken out of production, when the world is struggling to feed an ever-growing population?
Chris Gerrard, the Great Fen project manager, has two answers. The first is that the rich peat is vanishing fast. Not only does it shrink when it dries out, but in hot summers it simply blows away. Already in places the underlying clay is showing through.
That's still decent farming soil, but nowhere near as good. They'll be able to go on farming potatoes here for only another 20 or 30 years.
When I ask Farmer Papworth if he agrees, he gives a wonderfully non-committal answer, "Some say yes, some say no"; but it's not a question that need trouble him any longer.
The second answer is that protecting biodiversity and providing recreational access - especially for the rapidly growing city of Peterborough just to the north - are equally "productive" uses of the land.
The Great Fen project, it seems, could not only safeguard the fen violet's future, but redefine the priorities society gives to food production, conservation and tourism.
The BBC's Springwatch Season 2007 begins next Monday, 28 May, on BBC Two, and runs for three weeks. Bill Oddie, Kate Humble and Simon King will be reporting from across the UK on British wildlife.
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