People are more worried than ever about the possibility of an oil spill from Enbridge's Line 5 pipeline under the Straits of Mackinac, after the company caused the worst inland oil spill in U.S. history when another of its pipelines, 6b, ruptured in 2010, spilling at least 800,000 gallons of tar sands oil into the Kalamazoo River. The cleanup took three years.

Enbridge Energy’s Line 5 oil pipeline has lain deep under the water in the choppy Straits of Mackinac for more than 60 years.

Over the decades, Line 5 has fed billions of barrels of light crude oil and natural gas liquids into the lower peninsula of Michigan.

Yet there has never been a drill to test the region’s readiness for a spill from a leak or rupture of the pipe.

Until now, that is.

Enbridge, and ten local, state and federal agencies, including the U.S. Coast Guard, Mackinac County Emergency Response, and the Michigan Department of Environmental Quality held an oil spill response exercise Thursday. About 500 people took part in the exercise.

Setting up a command center not so easy in a tiny town

The small town of St. Ignace has few places that can suddenly turn into a command center for a disaster drill. Enbridge Energy sets up at the town’s ice arena.

The company's Stephen Lord says the aim was to make the situation feel real for the people working at command central. Make the spill big and add new situations throughout the day.

"Basically the scenario is fictitious," says Lord. "It’s highly, highly unlikely. It’s an oil release at depth in the Straits of Mackinac."

The amount of oil spill envisioned for the drill - 4,500 barrels – is a fraction of what Line 6b spilled into the Kalamazoo River five years ago.

One of the main reasons that spill was so large is pipeline operators at Enbridge headquarters in Calgary mistook a rupture in the pipe for a bubble of vapor in the pipeline.

It was 17 hours before Enbridge and other emergency responders began gathering at the site of the rupture, near Marshall, Michigan. By then, more than 800,000 gallons of oil had spilled into the river. It took three years for the cleanup.

Enbridge officials say that event changed everything about how the company approached safety and oil spills.

Out on the water

As helicopters buzz overhead, Charles Usher hops on a chartered boat and heads out to an open water exercise, to explain what's going on to media from the lower peninsula joining colleagues from the U.P.

Usher runs a private oil spill response company called Marine Pollution Control. His company would most likely be one of the first contractors to arrive, after the Coast Guard.

He points out the tugboat near the Mackinac Bridge, behind a long trail of orange booms, pulling a device called a skimmer.

"You’re basically driving it at the slick to contain the oil inside the boom and alongside the vessel," says Usher, "and ultimately it's slurped up by the brushes on the skimmer."

Within a few hours, a few such vessels could be on the water. Within 24 hours, dozens. But no matter how fast the response, Usher says most of an oil spill is driven to the shore by the wind and waves.

We've got that oil exactly where we want it

It's very difficult to recover oil on the open water, a little less difficult near the shore, where it can be funneled into little bays and collected.

Mike Paradise is a pipeline maintenance supervisor for Enbridge. He's stationed near a shore that is half-marsh, half sandy beach. The truck filled with big bags of coiled cottony stuff is always here, he says. Similar trucks are stationed at key spots where Enbridge has a pipeline.

"That right there is what we call 'sorbent boom,'" says Paradise. "It’s actually absorbent material, it doesn’t absorb water but it absorbs oil, all kinds of petroleums."

Paradise takes this drill very seriously. But he says his main job is preventing a spill by maintaining the pipeline. He says as long as Enbridge uses the right technology, the right way, Line 5 can remain as pristine as the day it was put under the straits.

"In my opinion the condition of the pipe is…is….great," he proclaims.

The pipe under the water is significantly thicker than most oil pipelines -- about an inch thick in many places. It's coated with enamel to reduce corrosion. And the pipeline can withstand pressure about 15 times the pressure at which the oil is actually pumped.

So I ask him, "How long could this pipeline last? Decades? 100 years? 200?"

"I would say yes, endless," he says.

Some environmental groups aren't buying it

Aaron Payment scoffs at the notion of the pipeline being so secure.

He's Chairman of the Sault St. Marie Tribe of Indians – one of the groups that thinks Enbridge Energy can’t be trusted and that the true condition of the pipe is being kept secret. He thinks no one could clean up a spill if the straits have frozen over. A spill is "inevitable," he says. "It's imminent that this is going to happen."

Payment and other environmental activists want the state to begin to plan for a shutdown of Line 5.

Enbridge: we have a different mindset now

Enbridge appears to be making no headway with these activists. But company President Al Monaco hopes that residents here, and those along the Kalamazoo River are getting the message, that the Marshall disaster forever changed the company’s approach to safety.

"Now where there is a doubt, where you’re not sure, let’s assume it’s a leak," says Monaco. "Let’s assume it’s a bad outcome. Shut down the system and then let’s figure it out."

Which is what the company has done on at least two occasions since the Kalamazoo River spill, Monaco says.

As a symbol of the company’s determination to avoid another spill, each employee is now given a ring, shaped like a small section of pipe.

It’s made from steel recovered from the ruptured section of line 6b.

In full disclosure, Enbridge energy is a financial supporter of this station.

Correction: An earlier version of this story reported Enbridge Line 5 transports liquid natural gas. That is not correct. it transports light crude and natural gas liquids.