The St. Lawrence Seaway is a monumental stairway in water, lifting massive ships hundreds of feet over thousands of miles. It’s the world’s longest inland waterway, a system of rivers, lakes, canals, dams, and locks that stretches 2,400 miles. And it’s one of the greatest engineering triumphs of the 20th century, pulled off against the violence of raging water and extreme winter. An essential part of the commercial infrastructure of the US and Canada, this complex system provides direct access from the Atlantic to North America’s heartland, enabling ships packed with trade to stop at any one its 65 ports–from Montreal to Duluth. From the 16th century, when French explorer Jacques Cartier searched for the legendary Northwest Passage, to the modern Seaway, built in the 1950s, we highlight the incredible engineering feats that went into creating the waterway.