Rockall is a stormy crag 240 miles off the Orkneys, and the last land grab of the British empire. John Vidal tells of his personal attachment to it – and why it could be worth £100bn

The following correction was printed in the Guardian's Corrections and clarifications column, Tuesday 4 January 2011

In the piece below headlined "Hello Mum, I'm on Rockall", this famous bit of granite in the north Atlantic was situated 240 miles west of Orkney. Rockall lies roughly that distance west of the Outer Hebrides.

On 15 September 1955, three marines and a civilian scientist from the Royal Navy's new survey ship HMS Vidal were winched from a helicopter on to a tiny, pyramid-shaped outcrop of granite sticking out of the Atlantic Ocean 240 miles west of the Orkneys. It was the height of the cold war and their secret mission was to annex the uninhabitable islet of Rockall and claim it as the last land grab of the British Empire.

Witnessed only by a few gannets and sooty fulmars, Sergeant Brian Peel, Lieutenant Commander Desmond Scott, Corporal Anthony Fraser and the naturalist James Fisher mixed buckets of cement and erected a flagpole made from old propeller shafts. Then they bolted a brass plaque commemorating the event to the rock, raised the union flag – standing back carefully in case they fell into the sea – and saluted.

"In the name of Her Majesty Queen Elizabeth II, I hereby take possession of this Island of Rockall," said Scott, becoming the last of a long line of sailors to formally claim a remote bit of the world for Britain.

But did he? Today, Britain, Ireland, Denmark and Iceland all claim Rockall, and within weeks the UN will examine rival claims to the mining and fishing rights of thousands of square miles of seabed around it. The final decision, expected by 2012, hangs on whether Rockall is geologically part of the continental shelf, as well as on historical records.

In 1955, the British were only interested in Rockall's strategic importance. The fear was that the Soviet Union could use it to spy on British nuclear tests, to be carried out from an experimental missile station on South Uist. These days, legal possession of Rockall could be worth £100bn or more, because it sits in the middle of a potentially vast oil and gas field. In times past, Britain would have gone to war over it; today a compromise to share it is most likely.

My own relationship with the lonely rock began in 1957 when my family was summoned by the navy for lunch on HMS Vidal, then berthed in Hull. We were supposedly descended from the great Captain – later Vice Admiral – Emeric Vidal RN, a brilliant young surveyor who mapped much of the Indian and Atlantic Oceans and who, in 1831, pinpointed Rockall for the first time at 57 deg 35 min N, 13 deg 41 min W. HMS Vidal was named after him.

As our adult relations drank pink gin with Captain Richard Connell, we children explored. Years later we discovered the navy had probably got their Vidals muddled up. Our side of the family were more likely descendents of the less salubrious Captain Charles Vidal RN, a slave owner and sugarcane grower in Jamaica.

For the next 40 years, Rockall went off my radar. The name was intoned every night on the shipping news; HMS Vidal was broken up in 1975 after an epic voyage taking scientists to the US nuclear station in Diego Garcia in the Indian Ocean. Its bell is reputedly with the family.

But then, in June 1997, the rock itself beckoned. Greenpeace was planning a landing on Rockall, claiming it as a micro-nation, renaming it Waveland in a stunt to resist the companies which were, even then, circling the seas around it in search of oil. Until then, more people had landed on the moon than on Rockall. I was invited along to become one of only a handful of people ever to have spent the night on the island.

We steamed due west and suddenly Rockall loomed out of the evening sea mists. The seas around the isolated crag are famous. In a gale, the waves crash right over it, sending spray 60ft or more above. Even on calm summer days the mid-Atlantic swell rises and falls 30ft and the sea laps and sucks heavily against Rockall's dark, barnacle-encrusted flanks.

We decanted from the MV Greenpeace into a small inflatable and approached warily. From a distance Rockall is a minute speck in a giant sea; from below it is formidable, with no obvious landing spot or route up. A six-inch wide potholing ladder had been lowered, and I was instructed to wait until the dinghy was at its highest point on the swell, then to leap out of the boat and grab the ladder. There was just one chance of getting it right. The ladder snaked down, the boat rose, I jumped and clung on, before clambering upwards. Ten minutes later, knuckles grazed and still in an extreme state of fear, I reached the top, and was violently sick.

There was not much to Rockall beyond a small ledge and a summit. In an act of vandalism, the navy had blown this off in 1971 to instal a beacon, which probably never worked. There was what looked like a shell hole on the western flank, made when it was used it for target practice in the 19th century.

But that was all. There was no soil, no fresh water, and our only companions were the birds. Seventy feet below us, a few seals played on the reef. We thought we saw a whale, but may have been mistaken.

"By claiming Rockall, we claim her seas for the planet and all the world's peoples. No one has the right to unleash this oil into our threatened climate," said my Greenpeace companion.

I telephoned my mother on a satellite phone: "Hello Mum, I'm on Rockall."

"Where?"

"Rockall. In the Atlantic."

"I know where it is. Are you quite mad?"

We considered tossing the British plaque commemorating HMS Vidal's annexation of the rock into the sea as a symbol of the end of empire, but in the end we just unscrewed it and turned it back to front. Greenpeace later put its own beside it. It read: "Let the sun and wind do their work. Leave the oil beneath the waves." I am told that the British plaque has now disappeared.

I am still a citizen of Waveland, and I have a small chunk of what James Fisher called the "loneliest islet in all the world's seas".

The last piece of the British empire is black, sea-smooth and hard as iron. It still smells of the Atlantic and is rarer than moon rock.