Eivind Grov, a senior adviser with Norwegian research group Sintef, estimated the cost of a tunnel between the island of Rousay and the Orkney mainland at around £20 million.
The total cost of any tunnelling project could rise to £130 million if a tunnel from the Orkney mainland to the island of Shapinsay and one to the Scottish mainland were added into the equation, he said.
Orkney Islands Council is currently examining the potential social and economic impacts of fixed links with the mainland and between islands.
A report published in February said that "a fixed link may enable the continued provision to an island of a service which would otherwise disappear", and that providing "key services to small islands is a critical factor in stemming depopulation".
But it warned that a fixed link can harm an island's economy if local services, such as shopping, can be provided from outside. A fixed link could also mean that Orkney is no longer considered an island, possibly rendering it ineligible for certain benefits, the report said.
During his visit, Grov examined sites linked with the proposed tunnels and made his presentation to representatives from Orkney Islands, Argyll and Bute, and Highland councils.
Bob Sclater, chairman of the committee examining the issue, said tunnels seem to be the way ahead. "There would be no restriction on travel and the costs don't seem to be astronomical. Fixed links provide obvious benefits to the islands, and it is the council's policy to consider improvements to our transport links - so the further investigation of tunnels is consistent with policy," he said.
A spokesman for Orkney Islands Council said the council was initially focusing on possible tunnel links within the Orkneys rather than links to the Scottish mainland. He said that, as well as boosting tourism, tunnels could link remote communities to services such as health centres on the Orkney mainland.
"The (tunnel) idea has been on the go for a few years," he said. The report said the council is examining alternatives to the current ferry service because of the expense of ferry replacement, their limited service and their susceptibility to delays and cancellations in poor weather.