A freshwater channel that separates Michigan’s upper and lower peninsulas is a premier Midwestern tourist attraction and a photographer’s delight, offering spectacular vistas of two Great Lakes, several islands and one of the world’s longest suspension bridges.

But nowadays the Straits of Mackinac is drawing attention for something that is out of sight and usually out of mind, and which some consider a symbol of the dangers lurking in the nation’s sprawling web of buried oil and natural gas pipelines.

Stretched across the bottom of the waterway at depths reaching 270 feet are two 20-inch pipes that carry nearly 23 million gallons of crude oil daily. They are part of the 1,900-mile Lakehead network, which originates in North Dakota near the Canadian border. A segment known as Line 5 slices through northern Wisconsin and Michigan’s Upper Peninsula before ducking beneath the Straits of Mackinac and winding up in Sarnia, Ontario.

The pipes were laid in 1953. They’ve never leaked, according to the system’s owner, Enbridge Energy Partners LP, which says the lines are in good shape and pose no threat.

But a growing chorus of activists and members of Congress is demanding closer scrutiny as stepped-up production in North Dakota’s Bakken region and Canada’s Alberta tar sands boosts the amount of oil coursing through pipelines crossing the nation’s heartland.

Concern has risen in the past year following serious spills in Arkansas and North Dakota, and as the government weighs the proposed Keystone pipeline project that would stretch from Canada to the Gulf of Mexico. The issue is especially sensitive in Michigan, where another Enbridge line ruptured in 2010, spewing more than 840,000 gallons of crude into the Kalamazoo River and a tributary creek.

The Straits of Mackinac epitomizes a potential worst-case scenario for a pipeline accident: an iconic waterway, ecologically and economically significant, that could be fiendishly hard to clean up because of swift currents and deep water that’s often covered with ice several months a year.

The 5-mile-wide straits link Lakes Huron and Michigan and flow near Mackinac Island, which is famed for its horse-drawn carriages and fudge shops. Several villages draw drinking water from the straits and cargo freighters and passenger ferries use it as a passageway. Sport anglers chase salmon and trout, while commercial crews harvest whitefish and perch for restaurants.

Hundreds of activists attended a rally to protest the pipeline last summer. Local residents haven’t paid it much attention over the years, but a packed crowd grilled Enbridge representatives at a community meeting this month.

“It’s a huge pipeline carrying oil in one of the most ecologically beneficial and sensitive places in the world,” said Andy Buchsbaum, director of the National Wildlife Federation’s Great Lakes office. “A massive oil spill there would have dire and irreversible consequences.”

The Senate’s second-ranking Democrat, Dick Durbin of Illinois, and Michigan Sens. Carl Levin and Debbie Stabenow sent a letter of concern to the federal Pipeline and Hazardous Materials Safety Administration in December. Agency head Cynthia Quarterman said Enbridge has agreed to step up its inspections of the Lakehead system since the Kalamazoo River spill.

“We’ve invested a lot of money, time and resources to ensure that we’re using the best available technology to operate our pipelines with the utmost integrity,” said Jackie Guthrie, spokeswoman for the company, based in Calgary, Alberta.

Now, line 5’s segment beneath the straits is getting extra attention.

Enbridge has reached an agreement with Michigan Technological University to deploy a newly developed “autonomous underwater vehicle” to provide digital images of the pipeline eight times in the next two years. The device resembles a 7-foot-long missile with a tiny, whirring propeller and will be fitted with sonar devices, cameras and computers.

The equipment probably isn’t capable of detecting cracks, but “never before have you been able to see this kind of detail,” said Guy Meadows, a director of the university’s Great Lakes Research Center.

The National Wildlife Federation maintains it’s time to replace the lines. The group posted a short video taken by divers that appears to show broken supports and sections suspended above the bottom or covered with debris. Critics also complain the company won’t release enough data from its inspections of the pipelines and note that above-ground sections of Line 5 have ruptured in numerous spots on land, spilling hundreds of thousands of gallons.

But Guthrie said the underwater section is sound.

“It’s been operating there for decades and operating safely,” she said.

The U.S. Coast Guard has conducted spill-response exercises in the straits the past three years. Some have taken place in winter to test technology for tracing oil beneath ice, said Steve Keck, a contingency specialist based in Saul Ste. Marie.

Dean Reid, planning commission chairman in Mackinac County who organized the community meeting, said locals needed more information about the pipelines, which many didn’t know existed until recently.

“We tend to take for granted what’s here,” Reid said, “and sometimes don’t know what’s here.”