In the summer of 1987, the American swimmer Lynne Cox braved the frigid waters of the Bering Strait to swim from the United States to the Soviet Union. Twenty-five years on, now aged 55, she recalls how her actions in the waning days of the Cold War eased international tensions.

"I wanted to open the border so we could become friends," says Cox. "The difficulty was that nobody believed it could happen."

Her route between Little Diomede Island, in the US state of Alaska, and Big Diomede Island, in the Soviet Union, was just 4.3km (2.7 miles) but it crossed the maritime border of two countries still locked in Cold War opposition, and the water was cold, very cold.

"There was this instant loss of breath," Cox recalls. "The cold was like a huge vampire pulling the heat from my body. I looked down at my fingers and they were totally grey, like the hands of a cadaver."

With the water temperature at 3.3C, the only way for Cox, then 30 years old, to survive was to keep moving.

"I put my face in the water and started swimming as fast as I could. I was also looking at my shoulders to see if they were turning blue because that would be really dangerous."

Cox first had the idea of the Bering Strait swim in 1976 and spent years lobbying Soviet officials for permission to enter their waters.

After being ignored at every turn, Cox finally decided to use "every last penny" of her savings to do her swim.

On the eve of the swim, there was still no word from Moscow, and the military on both sides of the Cold War were jittery.

"We knew something was happening because the Soviets moved two ships the size of football fields up into the Bering Strait," Cox recalls.

Image caption The Soviets had tea and biscuits ready for Cox and her support team

"The (indigenous) Inuits freaked out so they called the (US) National Guard and they sent up jet fighters. Then the Soviets sent up MiGs to check out why the Americans were up there. And I was thinking that this was supposed to be about world peace."

With 24 hours to go, permission came through from Moscow. President Gorbachev himself had seen a TV report about Cox's swim and - with the world's media watching - the Soviet leadership decided it would be too embarrassing to turn her back.

On the morning of 7 August, Cox woke up to find the Bering Strait completely calm. But there was no sign of the Inuit, who would guide her in their traditional kayaks.

"I'm all set to go and my crews are all set to go, but they're not up and I'm freaking out," says Cox.

It turned out that the Inuit had been up all night celebrating the prospect of seeing their relatives on Big Diomede for the first time in nearly 50 years. As they slept in, fog closed in and visibility dropped to 400m.

"We couldn't see anything, we didn't have radar, we had traditional canoes. Great Diomede is only 6.4km (4 miles) wide so everyone was really concerned that I might just miss the island."

Other Cox achievements 1971 - At 15, Cox swims English Channel, breaking men's and women's world records

1975 - first woman to swim across Cook Strait, New Zealand

1977 - first to swim 12.8 km round Cape of Good Hope

1987 - first to swim across Bering Strait

1992 - swam Lake Titicaca from Bolivia to Peru

As Cox started swimming, she was worried to see her support boats making constant changes of course. None of the Inuit was old enough to remember the route to Great Diomede and their only navigational device was a rusty compass.

In the end, one of the American journalists accompanying Cox intervened to put the expedition on the right bearing.

Cox then heard the sound of a motor. And slowly, a Soviet launch appeared.

"I was elated when I saw the skiff emerge from the fog - finally the Russians are here," she says.

On board was Vladimir McMillan, a half-American journalist for the Soviet news agency TASS, who was jumping up and down, shouting: "Lynne, don't stop now!"

Cox was heading for a cliff about 50m ahead, but with the fog clearing slightly, she could make out a Soviet delegation waiting further away on a beach.

McMillan wanted Cox to swim to the welcoming committee, but the American medical team urged her to take the easy option and swim to the cliff.

"I kept thinking 'I'm cold, I would like to finish this swim, but if I don't touch somebody's hand what have I done?'" she says. So she headed towards the Russians.

Image caption Cox now writes and talks about her open water swimming feats

The last 800m (0.5 mile) was the hardest part of the swim because of strong off-shore currents.

"I really did wonder how far I could go. I really did see my fingers go grey. Inside I was evaluating 'Am I OK? Can I keep going? Can I do it?'

"I had experts around me, but there's always the risk that you could go into cardiac arrest from hypothermia and it can happen really fast, so I was on edge that whole time."

The Soviet delegation came into view. Cox reached the shore, but it was so rocky she couldn't get out on her own.

"I extended my arm and two Russians in military uniform grabbed me," says Cox. "I instantly felt this heat from their warm hands. One guy was putting his arm underneath me to steady me. People were throwing blankets and coats on top of me. I didn't understand anything at all, except they were saying 'welcome'."

At the last minute, the Soviets had sent a top-level delegation, including KGB officials and sports stars. They had even prepared a small beach party.

Image caption Soviet President Gorbachev praised Cox during his US visit

"They had set up tables on the beach for a picnic with samovars full of tea and little biscuits. They were ready to celebrate all afternoon, but I was standing there on the ice thinking, 'Oh boy, this is getting cold.'"

Eventually, the Soviets let Cox go inside a tent to recover. A Soviet doctor, Rita Zakarova, covered Cox with hot-water bottles, put her in a sleeping bag, and then embraced her. For the American, the moment symbolised the entire trip.

"The whole idea was to have this human contact after so many years growing up afraid of the Soviets, and here was this person basically warming me up to get me back to life again," she says.

The swim turned Cox into a Cold War celebrity in the United States and the Soviet Union.

When President Gorbachev travelled to Washington to sign a nuclear weapons treaty later that year, he and President Reagan raised a glass to toast the swimmer.

"She proved by her courage how close to each other our peoples live," Gorbachev said.

Lynne Cox was interviewed on the BBC World Service's Witness programme. You can download a podcast of the programme or browse the archive.