A University of Washington instructor is proposing an underwater alternative to the state’s ferry system that involves tunneling several hundred feet below Puget Sound.

The idea of a tunnel running between Mukilteo and Clinton has been suggested by Bob Ortblad, a retired civil engineer who teaches a history of civic infrastructure class to undergrads at the UW.

His career has taken him all over the world, including to Iceland, where his fascination with tunnels under the sea began when he drove through one in Hvalfjörður

“I thought it was the coolest thing,” Ortblad said.

Over the past five years, he has used his combined background of accounting, engineering and economics to try to come up with ideas for subterranean passages that would gain traction with state transportation officials.

“It’s kind of just a fascinating problem,” Ortblad said.

His latest proposal — although it hasn’t yet caught the attention of many — focuses on an undersea route between Mukilteo and Clinton.

According to his designs, the tunnel would be four miles long and follow a curved path, rather than straight, to account for the depth of Puget Sound. It would be about 610 feet below sea level with a 6 percent grade.

Ortblad estimated the project to have a price tag of $200 million, basing the cost off of construction fees for similar tunnels in Norway and the Faroe Islands.

This cost could be covered by charging a toll to access the tunnel, he explained. It could be paid off within a decade if the toll is the same amount as the current ferry fare.

He predicted a tunnel would have nearly twice as many cars than the ferry route would.

The tunnel could have two lanes with several areas where emergency pullovers could be made in the event of an accident. An emergency tunnel could also be added.

A tunnel could also alleviate the pressure of traffic crossing Deception Pass Bridge. According to the state Department of Transportation website, 18,000 cars cross the bridge each day.

It might be an even greener option than the hybrid ferries. According to Ortblad’s calculations, a tunnel averaging 10,000 cars a day would require 2 million kilowatt-hours annually, compared to 33 million kilowatt-hours from electric ferries.

In addition, Ortblad said the passageway would have a lower operating cost and less environmental impact to marine wildlife.

While he acknowledged that he didn’t have the geology background to understand all the logistics of drilling such a tunnel, he suggested the sequential excavation method — which was used to create the Norway tunnel — could be used rather than tunnel-boring machines such as Seattle’s Bertha.

The hope is that the glacial till under Puget Sound is impervious, meaning it would keep the water out.

“That’s the big if,” Ortblad said.

A pilot tunnel might be considered to test the soil’s capabilities.

“If we don’t look, we don’t know,” he said.

Rep. Dave Paul, D-Oak Harbor, said he heard about Ortblad’s ideas for tunnels under Puget Sound.

“It’s an interesting idea — but there needs to be significant vetting of the proposal, including the cost of tunnel construction and the impact of adding a significant number of cars to both the Clinton and Mukilteo communities,” said Paul, who serves on the Transportation Committee. “I’ll be interested to hear what community members think of this idea.”

He added that he would want to avoid diverting resources away from the ferry system.

“I’ve really been focused on trying to work with Sen. Lovelett on trying to build up our ferry fleet,” Paul said, pointing to the plans of hybridizing the existing ferries by converting them from diesel to hybrid electric.

Ortblad said he has recently submitted his research and calculations to the Island County commissioners for their consideration.