If you walk so far onto England's southwest tip that one more step would plunge you over vertical cliffs into the Atlantic Ocean, you've probably found the world's most improbable theater.

"I don't believe this," exclaimed a recent playgoer at the Minack Theater, an open-air playhouse clawed from a straight-up-and-down cliff by the muscle and determination of one remarkable woman."You probably noticed the sign on the gate saying patrons proceed at their own risk," said artistic director Michael Dyer. The warning is no joke.

The Minack is so steep its very stairs zig-zag. Part of the hand rail is a rope bolted to native rock. The theater perches so precipitously that the Atlantic pounds into a chasm beneath its irregular stage. The backdrop to every play is the open, wide sea.

This year the Minack has a new clifftop exhibition center explaining the unlikely way that - as a simple granite plaque says - "Rowena Cade created this theater."

"It took us 50 years to do it," said Tom Angove, 74, leaning on a wartime pillbox which Cade, typically, converted into the box office.

Like the center, the 800-seat theater can be visited outside its Monday-Friday performances 16 weeks a year. Seeing it is hardly believing it - and attending a performance is simply unique.

This is England in the open air, so audiences clamber to concrete or grass seats laden with rugs, blankets and cushions. Even in July they wear stocking caps and thick sweaters.

Then the cliff becomes a picnic ground. Champagne corks pop, coffee pours into china cups from thermos flasks, silver cutlery appears from wicker hampers.

"We do Shakespeare, opera, ballet, musicals, every kind of serious play," said Dyer, artistic director since 1981. Most productions are by visiting amateur companies.

And no matter what the play, the setting is breathtaking. Huge rocks loom left and right. Ships and sailboats glide past. Cormorants skim the ocean surface. On rare nights a moon rises from the sea as stage lighting takes over from summer twilight.

Angove, regaling daytime visitors with Cade's exploits, is "the last one left" of the quartet that made the Minack. He, his uncle and gardner Billy Rawlings worked wonders, but Cade put them all to shame.

"Every grain of sand to make all this concrete, she hauled it up herself from that beach," Angove said, gesturing down to Porthcurno Cove, 90 backbreaking steps below. Cade built the steps, too.

Photographs in the exhibition show her as a small, seemingly frail woman of typical Victorian elegance. But her feats seem superhuman.

"Once she found some big square timbers, 15 feet long, on the beach there," Angove said. "She carried every one of them up here on her shoulders."

Cade and her mother moved to Porthcurno, a tiny village and beautiful cove on England's furthest southwest tip, in 1921. They built a house on the headland and Rowena became involved in amateur theatricals.

A 1929 Shakespeare performance in a nearby wood gave her the idea of a permanent open-air theater. Ready and waiting was her garden, if you can call it a garden. Cade once called it "a huge pile of boulders, a veritable rabbit warren."

But she and Rawlings laboriously hollowed out what now looks like a natural amphitheater. By 1932 they had carved a small stage and a terrace of grass seats for a performance of Shakespeare's "The Tempest."

A London Times reviewer happened to attend, and his report helped make the Minack permanent. Only World War II put it out of action, when the cliffs were festooned with barbed wire and the clifftop pillbox was built.

"Miss Cade used to crawl under the barbed wire with her lawn mower to keep the grass cut," Angove recalled. "She never let anything stop her."

Her lifelong obsession was building her theater, not appearing in it. She took its stage only once, a walk-on role in a 1935 production.

"She kept working well into her 80s," Angove said. Cade died in 1983 just before her 90th birthday. Shy and self-effacing, she liked to watch Minack plays from her favorite seat - an upended wheelbarrow.

"We get about 50,000 people a year," Dyer said. Tickets are 3 pounds ($5.10), and the theater survives without grants or subsidies. It is now run as a charity.