DETROIT – During the Prohibition era, Detroit's underground booze business exploded.

National Prohibition started in 1920, but Michigan actually banned alcohol three years prior, giving bootleggers a head start on the rest of the country.

Given Detroit's close proximity to an international port, Canada, it's no surprise how profitable illegal alcohol was for organized crime groups, like the Purple Gang, and speakeasy owners.

Related: When the 'Purple Gang' ruled Detroit

By 1929, making and distributing illegal alcohol, or rum-running, was Detroit's No. 2 industry behind automobiles.

The industry generated more than $300 million per year, which translates to more than $4 billion in today's economy. It's estimated that four-fifths of the nations supply of contraband booze came from Detroit.

What's rum-running

The term rum-running traditionally refers to smuggling over water, which was unique to Michigan. Bootleggers are traditionally associated with land smuggling.

Related: 6 Metro Detroit bars that used to be speakeasies during Prohibition era

Here's how it worked

Rum runners had several methods to smuggle booze into Detroit from Windsor.

One of the biggest methods was for large vessels docked in Windsor to have documentation for South America, but would then just drop illegal booze shipments in Detroit.

Rum runners would unload the booze into a boat at the docks, the boat driver would have a permit that would be stamped by a customs officer. In those days, many officers were being bribed.

In the winter months, rum runners would actually drive across the frozen Detroit river, in lighter vehicles dubbed "whiskey sixes," because of the smaller six-cylinder engines.

They also used a frozen Lake St. Clair for the same reasons. Here's a photo of a car falling through the ice during a smuggling attempt:

And when all else failed, the Purple Gang would just rob anyone who had booze in their vehicle, stopping drivers at gunpoint and stealing their loads.

Related: Prohibition ended 85 years ago today: Here's what it looked like in Detroit

How it all ended

Many blamed the Canadians for the huge volume of smuggling happening through the Detroit River. Canada didn't want to close their distilleries because of revenue and job creation and they didn't have the manpower to stop illegal activity in Windsor.

Detroit Border Patrol tried to stop the smuggling, but was largely unsuccessful. Then came the "Prohibition Navy." This new force had armed speedboats. But even they couldn't fully end the rampant crime.

Eventually, lawmakers argued the only way to end the violence and crime was to end Prohibition, which finally happened on Dec. 5, 1933.

Sources: Marty Gervais, “Rum Runners”, Bilioasis, Emeryville, Canada, 2009, Philip P. Mason, Rum-Running and the Roaring Twenties, Prohibition on the Michigan-Ontario Waterways, Wayne State University Press, Detroit, 1927