By the early 1850s the telegraph had stitched together the internal markets of continents, but the oceans still slowed the news down. A closing stock price from New York reached London in no less than ten days, even on the fastest steamer. The American entrepreneur Cyrus W. Field proposed laying a cable across the Atlantic in 1854; the Atlantic Telegraph Company was founded the same year. The British physicist William Thomson (later Lord Kelvin) joined as chief scientific adviser; the problems of signal attenuation and delay in long submarine cables became, for the first time, a direct object of physics research.

The first attempts ended in disaster. In 1857 the cable parted in mid-ocean. In 1858 two ships met in the middle of the Atlantic, spliced their cables and finished the run; a 98-word greeting from Queen Victoria to President Buchanan took 16 hours to transmit. Within weeks the line was burned out by the engineer Wildman Whitehouse, who applied excessive voltage. The American Civil War delayed the project by eight years. In 1865 the Great Eastern — designed by Brunel, then the largest ship ever built and the only one able to carry the entire cable in one load — set out with a new cable that again parted. In July 1866 she tried again, leaving on 13 July and reaching Heart's Content on 27 July with the line working. In September the lost 1865 cable was grappled up from the seabed, spliced, and completed: the Atlantic was now crossed by two working cables.

The effect was immediate. Before 1866 news of a London stock movement reached New York in an average of twelve days; after, in minutes. Reuters and the Associated Press began publishing transatlantic news on the same morning. The rhythm of diplomacy changed: foreign ministries could now tell their ambassadors "wait, instructions tomorrow" rather than "you are authorised, decide on the spot." By 1870 the cable network reached India, by 1871 Australia, by 1902 the whole world via the Pacific; by 1900 more than 300,000 km of submarine cable had been laid.

The historian Daniel Headrick called this network the "nervous system of empire": global communication was the precondition for both the global market and global power. The backbone of today's internet still consists of submarine cables; the optical fibres linking data centres are direct descendants of the copper conductor that Cyrus Field landed at Heart's Content in 1866. Humanity's habit of speaking to the far side of the ocean within seconds began on that July morning.