The roads, or "hollow ways", linked the valuable lead and silver mines of the Isle of Islay with ancient seaports then governed by the Lords of the Isles.
The 12th-century maritime kingdom was the economic and political power that controlled the Sea of the Hebrides down to the Irish Sea in the Middle Ages.
The find confirms that lead and silver were being mined far earlier than archaeologists thought, say researchers from Edinburgh University, who carried out the study with Nasa, the United States space agency.
Gary McKay, a Nasa scientist working with the geography and archaeology faculties, said the Shuttle radar identified the moist, densely packed ground under the roads, estimated to be up to 40ft wide. "It was so obvious something odd was present," Dr McKay said. "I thought it was a natural river course, then I realised this `river' ran up and over the hills, ignoring the geological structure. It had to be artificial."
One road from around Loch Finlaggan stretched to possible ports on the coast, known as Fionnphort and Port Bhoraraic. Another network surrounded Lochs Ballygrant and Lossit, which contain extensive deposits of lead and silver.
David Caldwell, curator of the National Museums of Scotland, said lead would have been used in building construction, and silver for trade and ornaments.
The Lords of the Isles, traditionally Macdonalds, dominated the region until their decline in the 15th century when the title was transferred to the Crown of Scotland, and later to the British Crown. Prince Charles is now Lord of the Isles. Many Scottish clans paid homage to the lords, who were treated as independent princes, Dr Caldwell said. "If you have a `Mac' in your name, the chances are your ancestors owed allegiances to them."
More sensitive radar instruments will be on future Shuttle missions in the hope of uncovering other sites of archaeological importance.
The radar would be able to decipher underground monuments to a resolution of between five and 50 metres, said Dr McKay, who traces his ancestry to Highlanders forced to emigrate to Georgia, in the US, in the 18th century. The Shuttle has already discovered details of archaeological sites in the Oman deserts and South American jungles.