The U.S. Army Corps of Engineers on Wednesday issued a permit for a tunnel that connects Lake Michigan and Lake Huron, bringing the project one step closer to construction.

The permit allows Canadian energy company Enbridge to build a tunnel underneath the lake bed in the Straits of Mackinac, where the two Great Lakes meet. The tunnel is designed to house a 4-mile segment of Enbridge’s Line 5 pipeline, which splits into two pipes across the bottom of the straits.