LAKE HURON — The Civil War was only seven months old on Nov. 9, 1861, when the sidewheel steamer Keystone State passed into oblivion.

Nobody even knew the ship had sunk until more than a week later, when wreckage was spotted off Port Austin and the Keystone State's wheelhouse washed ashore.

Since then, the ship has remained one of the many Great Lakes shipwreck mysteries; a tragedy that claimed the life of 33 crew members and sparked rumors about a clandestine cargo load of gold and war materials.

Divers explore the wreck of the Keystone State, a sidewheel steamer that sunk in Lake Huron in Nov. 1861. The wreck was found by David Trotter's team in July 2013.

This year, part of the mystery has been solved. On Monday, Dec. 9, shipwreck hunter David Trotter announced the discovery of the Keystone State in Lake Huron, about 50 miles north of Michigan’s thumb in less than 200 feet of water.

“She wasn’t where she was supposed to be,” said Trotter, an avid shipwreck sleuth with more than 100 discoveries to his name. “I probably thought I’d never find her.”

Trotter’s Undersea Research Associates team discovered the wreck in July using side-scan sonar and has since made several dozen dives to document the site and attempt to answer questions about the ship’s mysterious cargo, which some believe was intentionally mislabeled on the manifest.

The 288-foot-long Keystone State, luxurious for her day, was the second largest ship on the Great Lakes when she was launched in 1849 and is one of the largest side-wheel steamers to disappear into their depths.

The ship was bound for Milwaukee, Wis., when she left Detroit on Nov. 8, 1861, carrying what was labeled “iron implements,” or farm machinery, on the cargo manifest. Since the sinking, rumors have persisted that the Keystone State was actually carrying military supplies destined for the battlefield.

A cargo of farm machinery in November on a special run — the ship normally moved between Detroit and Buffalo, N.Y. — invites a natural suspicion, Trotter said.

“Farm implements are not heavily used in the winter,” he said.

Unfortunately, Trotter’s team found an empty cargo hold. The reason probably won’t ever be fully known, but he said ship crews of the time would likely dump cargo in an effort to save a vessel in dire straits.

The ship left Detroit in a hurry, without any lifeboats, he said.

According to the historical record, the Keystone State was last seen off Port Austin rolling heavily in rough water. The fact that she was found further north of where most believed she’d sunk “tells you she made quite a fight of it,” he said.

The ship’s captain, Wilkes Travers, may have been reluctant to turn the ship toward land for fear of being capsized in a sea trough, Trotter said. Control of a sidewheel steamer would be difficult in heavy seas due to the ship’s design.

Trotter said the wreck has “settled-in” quite a bit, and his team is still trying to sort out how much damage was caused by the storm, how much happened when the hull hit the bottom and what has occurred over the last 152 years.

The team hasn’t found any gold yet, but the wreck is surrounded by a large, yet-to-be-explored debris field, he said.

Due to the water depth, divers only have about 15 to 20 minutes to explore the wreck before they must decompress for more than an hour on the way back up.

Trotter, a retired Ford Motor Co. executive who lives in Canton, has been shipwreck hunting for more than 35 years.

His recent Great Lakes shipwreck discoveries include the 238-foot steamer New York about 25 miles northwest of Harrisville in July 2012, and a joint discovery with the Michigan Shipwreck Research Association of a 90-foot double-masted schooner in deep water off the coast of Grand Haven in October 2011.

“The most exciting shipwreck I’ve ever found is the next one,” he said. “There’s an exciting sense of discovery about being able to touch historical events that fascinates all of us.”

Garret Ellison covers business, government and breaking news for MLive/The Grand Rapids Press. Email him at gellison@mlive.com or follow on Twitter & Instagram