On 23 June 1870, a cable laid by the steamship Hibernia between Carcavelos near Lisbon and the beach at Porthcurno completed the last link in a wire that ran from London to Bombay. That evening, telegraph operators in India sent a test message westward. It arrived in real time at the Arlington Street townhouse of John Pender, the financier who had built the network, who was hosting a party for the future King Edward VII. The signal was read off a siphon recorder, an instrument invented by Sir William Thomson, the Glasgow scientist who would later become Lord Kelvin. The ink traces it produced were the new long-distance handwriting of the world. Within fifty years more than a dozen cables had landed at Porthcurno, connecting Britain to Gibraltar, Malta, Newfoundland, Cape Town, Aden, Singapore, Hong Kong, and onward. The beach handled up to two million words a day. The Empire spoke through this sand.

Porthcurno was an obvious target. A single German bomb on the cable hut could blind half the Empire at a stroke. So in June 1940, with invasion forces gathering on the French coast, Cornish tin miners began driving two parallel tunnels into the granite of the valley's eastern wall. They worked by candlelight and pneumatic drill, joined the parallels with cross-tunnels, and finished with a fifth steeply ascending escape passage that emerged in a field above. Each tunnel mouth was guarded by offset double doors, bomb-proof and gas-proof. A local artist designed camouflage for the concrete entrance defences that, viewed from a German reconnaissance camera, was supposed to resemble a belt of trees complete with rabbits and birds. The deception was probably less effective than the doors. Inside, the equipment hummed at a constant temperature in dry granite that the engineers came to love. After the war they left it there. It worked too well to move.

The museum's collection has been awarded designated status by the Arts Council, which means the government has formally recognised it as nationally important. The holdings include the archives of Cable and Wireless plc, the company that grew out of the 1934 merger of the original Eastern Telegraph Company with Marconi's Wireless Telegraph Company. There are working examples of telegraph equipment, models of cable ships, paintings, samples of every generation of submarine cable from gutta-percha through coaxial copper to optical fibre, and one section of TAT-1, the first transatlantic telephone cable, completed in 1956. There is also an axe. It belonged to a landing party from the German cruiser SMS Emden, which on 9 November 1914 went ashore at Direction Island in the Cocos Islands and chopped through three British cables before HMAS Sydney showed up and finished the Emden's career.

The cable office at Porthcurno closed in 1970. The training college that succeeded it stayed open until 1993, then shut its doors as the industry consolidated and the work moved elsewhere. Former employees, refusing to let the story die with the company, started a small museum at the Cable and Wireless Holborn headquarters in London and gradually relocated it back to the tunnels where it belonged. It opened to the public in May 1998. The Princess Royal returned in 2010 to open the Nerve Centre of Empire exhibition, designed by local artist Hennie Haworth and funded partly by Exeter University. In March 2022 Neil MacGregor devoted an episode of his Radio 4 series The Museums That Make Us to PK Porthcurno. He understood what the place was. Long before the internet existed there was an internet, and it ran through this cove.