In the 1950s new technology put cables ahead of radio. Small vacuum tubes that could operate under water for 20 years or more meant that amplifiers could be buried at sea with the cable. This boosted the cable's information capacity to the point that it could even carry telephone signals. Small vacuum tubes like this could be buried at sea with the cable for years. They helped to increase a cable's information-carrying capacity by more than a thousandfold.

Vacuum tube, 1958 National Museum of American History, from TyCom

In 1956 AT&T teamed up with the British General Post Office to lay two cables across the Atlantic, each transmitting in a single direction. Together they could carry 36 telephone channels, and this soon expanded to 48. Other telephone cables soon followed, and the old telegraph cables became obsolete. The new cables had a central copper conductor surrounded by a second coaxial conductor that provided a "return" path for the electricity. Instead of gutta percha, the insulator was polyethylene, a synthetic plastic that had been developed in the 1930s.

TAT-1, the first transatlantic telephone cable, 1956 National Museum of American History, from Robert Lynch, Director, System Implementation, TyCom

Laying telephone cable by ship, 1960s Photograph by Ian Rowan

Transatlantic telephone cables Courtesy of Corning, Inc.
TAT-1	1956 - 48 channels Nefoundland - Scotland
TAT-2	1959 - 48 channels Newfoundland - France
CANTAT	1961 - 80 channels Newfoundland - Scotland
SCOTICE-ICECAN	1961-62 - 24 channels Newfoundland - Scotland
TAT-3	1963 - 138 channels New Jersey - England
TAT-4	1965 - 138 channels New Jersey - France
TAT-5	1970 - 845 channels Rhode Island - Spain
CANTAT-2	1974 - 1,840 channels Nova Scotia - England
TAT-6	1976 - 4,000 channels Rhode Island - France
TAT-7	1978 - 4,000 channels New Jersey - England

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