Modern Marvels: In the land of Mardi Gras, jambalaya, and zydeco, exits an engineering marvel called the Lake Pontchartrain Causeway that seems to go on forever. Two ribbons of concrete span the largest inland body of water in Louisiana, and at nearly 23.87 and 23.88 miles long, these two spans form the world's longest automobile bridge. At midpoint--12 miles out--water surrounds travelers who are unable to see either shoreline. The bridge is so long, it actually transverses 1/1000th of the earth's circumference!