Kimberly Hatfield

Not too long ago, when Tamara Thomsen—maritime archaeologist with the Wisconsin Historical Society in Madison—would dive down to the Rouse Simmons shipwreck, the scene was worthy of a Disney movie. (Known as the “Christmas Tree Ship”, the 205-ton, three-masted schooner took a nosedive into Lake Michigan in a storm in 1912 with a hold full of Christmas trees.) As Thomsen descended, the ship would come into view, its hull laid bare with the decking and rigging flung towards the area of impact. Fractured trestles revealed the cargo hold still stacked hundreds deep with trees. At a depth of around 50 metres, the cold water held the ship suspended in time. Thomsen could see the ship’s signature blue, green and red paint and, if she got close enough, the needles still on its Christmas trees.

Those rich details, from the design of the vessel to its contents and its ultimate demise, were once a lasting record of the commercial and human history of this ship in the early 20th century. But now, they are in dire risk due to the invasive quagga mussel that is obliterating the ship’s surface.

What lies beneath

The Great Lakes straddle the US-Canadian border, hold around 21% of the world’s surface fresh water and link to the Atlantic Ocean. Below the surface are some of the most historically important and well-preserved submerged archaeological sites in the world, which tell the rich stories of prehistoric human activity, the western expansion of the US and the thriving Great Lakes region.

The underwater discoveries range from a 9,000-year-old caribou hunting structure in Lake Huron to modern aircraft. They also include countless shipwrecks that chronicle the evolving history of shipbuilding and commerce, from early wooden schooners to iron-hulled steamers and ferry-borne rail cars—some still on their tracks and others filled with Kohler bathroom fixtures.

These historic remains have been remarkably well preserved because of the unique waters of the region that prevent the decay of human-made structures, says Daria E. Merwin, a co-director of the cultural resource survey programme at the New York State Museum in Albany. Many of these relics are at “a depth where there’s not a lot of microbial activity, there’s not as much oxygen or light”, she says. “The water is cold, and it tends to stay at a constant temperature.”

However, the construction of the St Lawrence Seaway in the 1950s, which enabled access to the lakes for ocean-going cargo ships, brought a new variable to submerged cultural sites in Great Lakes waters: invasive species in the ballast water held in the tanks and holds of ships for stability. Hitchhiking zebra and quagga mussels, related types of mollusc native to the Caspian and Black Seas, were first detected in the Great Lakes in the 1980s. Zebra mussels had been well studied and are easily recognised, populating shorelines and shipwrecks, and famously defacing the once pristine War of 1812 ships the Hamilton and the Scourge in Lake Ontario. However, the unique adaptations of quagga mussels have allowed them to quietly outcompete zebra mussels in the region, with disastrous consequences for its deep-water heritage sites.

Voracious but patient filter feeders, quagga mussels have deftly adapted to the cold, spare conditions of the deeper waters of all the Great Lakes with the exception of northern Lake Superior. They blanket the lake bottom and colonise in reef-like layers on anything projecting from the floor to exploit small, nutritious currents. Their byssal threads, commonly referred to as the mussel’s “beard”, anchor to nearly any surface with an adhesion so strong that removing them damages what they are attached to. Even their simple act of breathing out carbon dioxide creates an acidic, corrosive microenvironment.

Despite their fingernail size as individuals, masses of quagga mussels can be heavy, says Lyubov E. Burlakova, a senior research scientist at the Great Lakes Center at Buffalo State University. She estimates that reefs of quagga mussels can weigh several kilograms per square metre, burdening already compromised structures.

The Wisconsin provides an example of the damage these molluscs can do. Built in 1881, this innovative iron-hulled ship counted among its crew the apprentice machinist Henry Ford. Submerged in Lake Michigan since 1929, the shipwreck has been explored by researchers and amateur divers for decades. Today, it is mired by mats of quagga mussels that have corroded the hull, cleaving the sides from the body.

“These early iron or steel vessels are really vulnerable to the mussels,” says Thomsen, who collects data to record the condition of the sites. “That shifted our priority here in the office. Whenever there’s a metal or steel ship that’s discovered, we respond right away, because they are going to be changing, and they’re really changing exponentially.”

Engines of discovery

Documenting known sites and finding new ones has always been expensive and time-consuming, especially in the Midwestern US, a region known for its harsh winters. However, new technologies for both seeing and being in the water are improving the odds of finding valuable artefacts. The latest tools include vehicles that can be remotely operated from land or underwater, sonar mapping technologies, and magnetometers which can detect metal objects—from small fasteners to cannons and anchors—beneath the lake floor.

Finding new sites is never quick or easy, says Carrie Sowden, the archaeological director at the National Museum of the Great Lakes in Cleveland. The process involves extensive historical research followed by an expedition known as “mowing the lawn”, a painstaking search for areas of interest by dragging sensing equipment through the water. “You go two knots an hour while you’re pulling towfish, most often a side-scan sonar, behind you,” Sowden says. “And you’re staring at a computer screen waiting for just the tiniest little blip to show up so that you can then go back around and zoom in.”

But even these advanced technologies have limitations. Merwin, who has surveyed the Hudson River for submerged archaeological areas, says an entire class of underwater sites, such as fish-trapping weirs, may be awaiting discovery. “Since we haven’t found them yet, we haven’t systematically gone out looking for them,” she says. “If the quagga mussels beat us to it, it’s going to make it so much harder.”

Compared to archaeological sites on land, which are often in a state of flux, submerged sites are rare gems, Merwin says. They represent a unique opportunity to look into the past, “almost like a moment frozen in time”. Quagga mussels now obscure that view, with no reliable population control in sight.