Answer: The completion of the first transatlantic telegraph cable
The cable, which ran from Ireland to Newfoundland, made possible the first real-time, intercontinental communcation. The cable was laid by the Atlantic Telegraph Company, a company created by American entreprenuer Cyrus West Field. The cable was opened for business on August 16, with an inagural telegram exchanged between Britain's Queen Victoria and U.S. president James Buchanan.
The cable was not long for this world, however. The following month, Wildman Whitehouse, the cable's chief engineer, inadvertently destroyed the cable by ramping up the voltage in an ill-advised attempt to repair it.