'I Hate the Undercliff." No, not me personally, but while shooting the French Lieutenant's Woman the film crew wore T-shirts bearing that message. They were referring to the difficulty of filming in the area to the west of Lyme Regis, where Meryl Streep's character had her trysts with an illicit lover played by Jeremy Irons in the 1981 box office hit.

And a stroll into this virtually subtropical landscape of dense trees, tumbling creepers, thick undergrowth plucking at your legs, and slippery tree roots to tumble over soon makes you realise that it wouldn't have been much fun lugging all the paraphernalia of a film shoot in here.

But there is no reason for you to hate this seven-mile stretch of Heritage Coast. In fact, John Fowles, author of the book on which the film was based and a local resident, says: "I treasure it for its solitude, its silences [strange that, because one of the things I noticed was the vibrant birdsong of the Undercliff], its sheer beauty, its exuberance of growth."

It is indeed a unique spot. It takes the best part of an afternoon to walk from the eastern end of the Undercliff at Lyme Regis to the rather drab seaside town of Seaton at the western end, and, unless it's the height of summer, you're unlikely to pass more than a handful of people on the way. Meanwhile, the beaches of Lyme will be busy with fossil collectors ambling along the seashore, heads down like sulking schoolboys as they search for the region's famous ammonites, and day-trippers idling through the town's steep winding streets or enjoying the views from the pretty, bustling harbour.

However, to leave the hubbub around Lyme Regis and head west into the Undercliff, you need to have a penchant for burrowing around beneath thick tree canopies. Since the route follows the coastline, I had preconceptions of cliff-top views of dramatic seascapes all along the way, but so unstable are the cliffs here that the path is actually too far inland to see much of the shoreline apart from at a couple of spots.

What you get instead is a twisting, turning, undulating trail that appears to be under constant threat of disappearing beneath a riot of jungle-like vegetation tussling for supremacy. A combination of rich soils, plenty of rain and south-facing slopes has resulted in the kind of landscape where you start to feel that a machete may be called for at any time. Yet, until a century ago, much of the area was worked by woodsmen, quarrymen, shepherds and pig herders, with various crops and fruits also grown here. Cricket matches were even held on the Undercliff, which beggars belief since I failed to see any patch of land flat enough to set up even a snooker table.

Geologically, the Undercliff is anything but stable. Great lumps of cliff fall off on occasion and head seawards. This process is best seen towards the western end of the walk at Goat Island, a 15-acre block of land that was left standing in isolation after some eight million tons of rock and soil slid away into the sea on Christmas Eve, 1839. A dry chasm 150ft deep and three-quarters of a mile long separates Goat Island from the "mainland", but because crops had been planted on the land before it sloped off seawards, the enterprising locals continued to cross the chasm and work their "fields", enjoying a ceremonial harvest in August 1840.

You can see examples of cliff falls at various intervals along the path, the fresh white chalk gleaming against the greenery clambering up the cliff sides and, as you emerge from the undergrowth just west of Goat Island to cross open fields, you'll notice fissures where the land is preparing to make another trip out to sea.

This combination of luxuriant vegetation, unstable slopes and few readily accessible areas has made the Undercliff a safe haven for flora and fauna, and in 1955 it was designated a National Nature Reserve. Flowers that are relatively rare elsewhere in the UK are quite common here, and include bee and marsh orchids, while the fauna includes roe deer, weasel, badger fox, adders, grass snakes, great spotted woodpeckers and treecreepers.

The narrow path through the Undercliff is as near as most people will get to any of this wildlife, but it's not really a good idea to wander off the beaten track anyway. Apart from the fact that you could disappear down a hidden fissure or sink hole, it's a hell of a job thrashing your way through the triffid-like plant life, and it's also very easy to get lost.

After I'd made the trek through the jungle, I have to say that I was glad to get out into the light and airy cliff tops above Seaton, but it's good to know that there are still places in the jam-packed south of England where nature remains in charge.

Way to go

Getting there:
It's easiest to drive to Lyme Regis because the nearest railway station is at Axminster, five miles to the north (call 08457 484950 for national rail enquiries). If you walk the Undercliff as far as Seaton, the X53 bus service (First Southern National 01823 272033) takes you from the seafront back to Lyme Regis.

Where to stay:
The friendly, comfortable Clappentail House (01297 445739) is a listed 17th-century building at the top of town, from £21pp B&B. The Bay Hotel (01297 442059) has a good sea-front location, convenient for the beaches, from £35 B&B.

Food and drink:
Lyme Regis has a wide range of cafes, pubs and restaurants. Rumours, 14/15 Monmouth St (01297 444740) serves excellent seafood, and the Cobb Arms (01297 443242), above the harbour, has decent pub grub in a lively atmosphere.

Further Information:
Tourist Information, Guildhall Cottages, Church St, Lyme Regis (01297 442138, westdorset.com).

Fossil information:
The local coastline has been designated a Natural World Heritage Site because of its outstanding geology, which offers an almost complete record of 185 million years of Earth's history. It is particularly famous for its fossils - you can see ammonites the size of manhole covers in situ on Monmouth beach, and dinosaur fossils and tracks have also been discovered locally (Jeremy Irons plays the part of a fossil collector in The French Lieutenant's Woman). You're almost certain to find your own souvenir fossils with a walk along the beaches either side of Lyme Regis, but if you don't have any luck, check out the examples in the Philpot Museum (lymeregismuseum.co.uk) or the town's fossil shops. Fossil walks are organised by Dinosaurland (01297 443541) and local geologist Dr Colin Dawes (01297 443758).