The Great Lakes, a vast expanse of fresh water, hold one-fifth of the world's supply, yet their origins remain shrouded in mystery. Scientists are now piecing together the dramatic, ancient history of these inland seas, uncovering evidence of a massive, long-vanished ice sheet that once carved out their basins. Through the study of fossilized remains and geological formations, researchers are revealing how these lakes were shaped by powerful, natural forces, and how they have evolved over millennia to become the vital, interconnected water system we know today.