What Enbridge Didn't Want You to See
Its engineers said "weak to very weak." Its lawyers said you couldn't see it. Now you can.
Picture drilling a nearly four-mile tunnel through rock so fractured that it has partially collapsed on itself, rock that Enbridge’s own engineers classify as ‘weak to very weak’ at pressures exceeding 250 pounds per square inch, beneath one of the world’s largest freshwater systems.
Now add an uncontrolled source of explosive methane gas. That, in summary, is what Enbridge’s long-concealed geotechnical report assesses: the proposed Line 5 tunnel through the Straits of Mackinac. The report’s long-delayed public disclosure is in a filing with the Michigan Department of Environment, Great Lakes, and Energy (EGLE), which is expected to issue a crucial decision on July 16 on a required permit to construct the proposed tunnel.
For years, Enbridge resisted releasing key sections of its Geotechnical Baseline Report on the proposed tunnel project. Even the version now in the public record is redacted in critical areas.
Enbridge says the report was written for contractor negotiations, not permitting. But EGLE had to request the document in late 2025, and it took Enbridge months to provide it. The report wasn’t part of the Michigan Public Service Commission permit record in 2023. A report describing the ground conditions of the tunnel route was withheld from permitting agencies for years. Draw your own conclusions.
But what remains, together with independent expert analysis, is sufficient to explain the company's reluctance to share it.
The Rock Enbridge is Betting On
A tunnel boring machine (TBM) needs stable, competent rock to grip, push against, and seal itself into. The Straits of Mackinac does not reliably offer that. Here is how geologist Mike Foley, who spent his career assessing project risks for the oil industry, summarizes what he read in the report:
"The formations they propose to tunnel through are described as a mixed lithologic assemblage of limestones, dolomites, and claystones, much of which has been heavily fractured and even brecciated — broken up — by dissolution of an underlying layer and subsequent collapse in the distant geologic past."
Nearly 70 percent of the proposed route runs through the Pointe Aux Chenes Formation, which is a highly fractured, incompetent claystone. ‘Incompetent’ is an engineering term of art: it means the material cannot reliably support stress without deforming, cracking, or collapsing. Enbridge’s own rock strength analysis classified most of it as ‘weak to very weak’ on the standard rock quality scale.
There is one location in particular that concerns Foley most. At the deepest point of the tunnel, where pressures will be highest, the planned route appears to come within just 30 feet of an ancient erosional channel filled with loose, unconsolidated glacial sediment.
Enbridge’s own report confirms an ancient riverbed — the “Mackinac River” — lies beneath the Straits, filled with soft, unstable sediments. Then it claims the tunnel won’t encounter it while simultaneously requiring engineers to prepare for exactly that scenario if conditions differ. But if the tunnel boring machine drifts even slightly from its planned path, or if the geology is more variable than sparse borehole data suggest, the machine could punch into that sediment zone. The consequences could be severe and potentially irreversible.
History offers a useful precedent. This is not speculation about what might go wrong but a record of what has gone wrong before.
In December 2013, a tunnel boring machine in Washington State struck an uncharted steel pipe, which was a single unexpected obstruction, and broke down. The project halted for roughly two years while crews excavated a rescue shaft and rebuilt the machine. Litigation over insurance costs dragged on for years afterward.
The Line 5 tunnel route relies on approximately 20 boreholes spread across nearly four miles, with gaps between sampling points often exceeding 1,000 feet. In some stretches, a single borehole hundreds of feet away is the primary evidence for what the ground looks like. That is an extraordinarily thin basis for predicting what a multi-million-dollar machine will encounter.
While the geotechnical report briefly references gas potential — with key sections redacted (see below)—independent tunnel expert Brian O’Mara raises a concern that goes far beyond what the report acknowledges. O’Mara has more than 35 years of geological engineering experience in tunnel construction. His assessment is direct: methane dissolved in groundwater will seep into any tunnel drilled through these formations. It will not be a temporary construction hazard. It will be ongoing and permanent.
This transforms the project from a difficult construction challenge into something more dangerous: a confined, high-pressure industrial tunnel with a continuous fuel source for an explosion. The danger threshold for methane is low. Just 5 percent of methane in the air creates an explosive mixture. Tunnels offer limited ventilation and no escape route for a pressure wave. The historical record on this is sobering:
1988 Milwaukee, Wisconsin. A methane explosion in a tunnel killed three workers. Earlier, near Port Huron, a similar tunnel explosion killed 22 workers — one of the deadliest tunnel disasters in American history.
A sealed annulus design, which would reduce methane infiltration, has not been adopted. The project also lacks a Failure Modes and Effects Analysis, a basic engineering safeguard that catalogs how individual system failures can cascade. Its absence is not a technicality. It means there is no systematic accounting of how one failure might trigger the next.
The methane risk does not stand alone. The geotechnical report itself identifies a series of conditions that would challenge any tunneling project and that interact with each other in ways that make collective failure more likely than any single risk in isolation.
Continuous pumping and water treatment would be required throughout construction. The report also acknowledges 'rapid failure' or 'fast raveling' as a potential condition. This is the sudden collapse of material into the tunnel bore, something extremely difficult to respond to in a confined space under high pressure.
What the Law Requires and What This Project Does
Michigan’s Great Lakes Submerged Lands Act requires that no activity impair public trust waters. The Clean Water Act requires that projects avoid and minimize impacts wherever possible. This project, as described in its own technical documentation, runs counter to both standards.
Violating the Great Lakes Submerged Lands Act, the project would disturb unstable geological formations, risk collapse and groundwater interaction, and require continuous industrial intervention throughout its operational life. That is not protection of the resource. It is a permanent risk management imposed upon it.
Violating the Clean Water Act, the project requires continuous water withdrawal and discharge, introduces chemical treatment systems into the environment, and poses a documented risk of contamination. In the worst case, O’Mara identifies — a methane explosion or structural tunnel failure — the tunnel could collapse, releasing oil or natural gas liquids directly into the Straits.
Mackinac Bridge Risk
The geologically mixed conditions, which include the ancient riverbed running beneath the Mackinac Bridge corridor, also raise a concern that has received little attention: the risk of a caving event affecting the bridge itself during nearby tunnel construction. The report does not appear to address this directly.
What This Report Reveals
● Geology that is fractured, weak, and partially collapsed from ancient dissolution events
● A critical proximity to an unconsolidated sediment channel at the deepest, highest-pressure point
● A continuous, permanent methane infiltration hazard with no sealed annulus mitigation
● Sparse borehole data — roughly 20 samples across 4 miles — insufficient to characterize conditions the machine will actually encounter
● No reliable excavation rate, meaning timeline and cost projections are essentially guesswork
● No Failure Modes and Effects Analysis — no systematic accounting for how failures cascade
● Continuous industrial intervention is required, permanently, as an operating condition
● Redactions in key sections of the report that would reveal the full scope of risk
You don't build a tunnel through collapsing rock, under crushing pressure, with a known methane source, beneath the most important freshwater system in North America, and call it an engineering solution. You call it what Enbridge's own report describes it as: a project defined by unknowns, managed by assumptions, and protected by redactions. EGLE's July 16 decision on whether to grant a permit for the proposed tunnel will determine whether Michigan accepts that framing or demands something better.
TAKE ACTION
Tell Governor Whitmer and EGLE Director Roos that we are watching and expect a decision denying the Enbridge tunnel permit. There is still time for you to act, and it only takes a minute.
Thanks for this detailed and incredibly disturbing breakdown of the Enbridge geotechnical report! It is time for EGLE to just say no to this illconceived plan, and shut down the existing pipelines before disaster happens.
Signed and will restack.