Deep under the Great Lakes, once America's maritime highway, shipwrecks litter the floor. Just off Wisconsin alone, it's estimated there are as many as 750 lost ships and most of their final resting places remain maritime mysteries.

But these discoveries, some 400 feet underwater in near freezing inky blackness, are now being threatened by an invasive species potentially more destructive than the ravages of time.

"I think the optimal window as far as getting video and seeing these things, is starting to close now," diver John Janzen said.

Mussels, zebra and quagga mussels, to be exact, have invaded the Lakes - and the wrecks themselves.

It's been happening since the late 1980's, but they've reached such numbers, archeologists fear they may now be doing real damage.

"We know that they're doing something to these wrecks," Caitlin Zant, a marine archeologist with the Wisconsin Historical Society, said. "But we're not entirely sure what yet."

Images from the wreck of the Kyle Spangler, for example, show the difference just five years can make. Mussels pile up - sometimes several inches thick.

Unlike the ocean - with its corrosive salt water - the Great Lakes have a reputation of entombing their wrecks in almost pristine condition. But some of the wrecks are already beginning to collapse and the fear is those mussels may one day reduce these silent sentinels to monuments no longer recognizable.