The tunnel, which would be bored into bedrock deep below the Straits between St. Ignace and Mackinaw City, will house a replacement segment for the dual Enbridge Line 5 petroleum pipelines that currently sit exposed on the bottom of the Straits. Environmentalists and a growing chorus of lawmakers have expressed fear that the existing dual 1950s-era pipelines could fail or get hit by a ship's anchor, resulting in a petroleum leak that would devastate the region.

Enbridge will pay for the tunnel, which is estimated to cost between $350 million and $500 million, and plans call for the 4.5 mile tunnel just west of the Mackinac Bridge to be operational between 2025 and 2028.

“This new law is the result of a tremendous effort by Sen. (Tom) Casperson (R-Escanaba), my partners in the House and Senate leadership, and a diverse group of stakeholders, including business leaders and union representatives,” Snyder said. “We all understand the importance of bringing certainty to removing Line 5 from the waters in the Straits of Mackinac. By working together, they helped garner bipartisan support to ensure we are protecting the Great Lakes while securing better energy infrastructure for Michigan.”

The newly appointed three-member authority on Dec. 19  approved an agreement with Enbridge to build a multi-use utility tunnel beneath the Straits of Mackinac.

Snyder's office said under agreements already signed between the state and Enbridge Energy, the company will pay for 100 percent of design, construction, operation, and maintenance of the tunnel for the next 99 years. The company also will decommission the twin pipelines in the Straits once the tunnel is complete. The project "will create good-paying jobs for workers in the professional trades and others while continuing to provide stable energy prices for Michiganders," Snyder said.

In a statement of support of the new tunnel to state lawmakers, Michigan Building and Construction Trades Council President Steve Claywell said it would likely be the largest infrastructure project in the Straits region since the construction of the Mackinac Bridge. He said the tunnel would be a huge boon to the region, with good-paying jobs for construction workers and spinoff benefits of more activity at restaurants and motels, even during the off seasons.

"The tunnel and the pipelines will create a  safe conduit to move energy products, and virtually eliminate any chance of a petroleum spill in the Straits,"  Claywell said. "Encased in concrete 100 feet below the Straits, the utility tunnel will also offer new space for reliable electrical, telecommunications and other utility services, providing a comprehensive solution to connect our two peninsulas while protecting the Great Lakes."

The tunnel will have the capacity to encase the Enbridge Line 5 - which moves petroleum products and natural gas liquids from Superior, Wisconsin, through Michigan to Sarnia, Ontario - as well as propane, electrical, and telecommunications lines. Line 5 is a key energy conduit for the state, currently delivering 65 percent of the propane used in the U.P., and 55 percent of Michigan’s statewide propane needs.

According to the Marquette Mining Journal, natural gas liquids from Line 5 are accessed at a propane facility in Rapid River in the U.P., and sold to local and regional distributors. Calls to shut down Line 5 created fears in the U.P. that making that reducing the supply would cause propane prices to spike.

"This is the best solution we need to protect our beautiful Great Lakes and ensure Northern Michigan families continue to have the resources they need to heat their homes," said House Speaker-elect Lee Chatfield, who represents Emmet and Mackinac counties where the oil pipeline crosses through the Straits of Mackinac. "This has been discussed for years, and doing nothing is not an option. It's time to move forward."

With newly elected Gov. Gretchen Whitmer opposed to the tunnel, the state's GOP majority moved quickly to get the authority in existence before the end of 2018. There are certain to be legal challenges to the pipeline project.