The Canadian company said Thursday its crews took core samples from 27 locations in the Straits of Mackinac area, where Lakes Huron and Michigan meet.
Project manager Amber Pastoor said geologists will study the samples. Results will be used to design the tunnel and a machine to drill through bedrock beneath the lake floor.
Enbridge negotiated an agreement with former Republican Gov. Rick Snyder to run an oil pipe through the tunnel and shut down its twin Line 5 pipes that traverse the lake bottom.
His Democratic successor, Gretchen Whitmer, opposes the deal. Attorney General Dana Nessel is appealing a court ruling that upheld legislation authorizing the agreement.
Nessel also has filed a lawsuit to decommission Line 5.
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