Follow me to certain death.
When Captain Garcia Lopez de Cardenas and his men, of the Spanish exploration party led by Francisco Vásquez de Coronado, became the first Europeans to see the Grand Canyon, in September of 1540, they estimated the Colorado River at the bottom of the canyon to be eight feet wide. (It is actually 200-300 feet wide.)
Interesting Fact of the Day:
The point is that even with their eyes wide open, standing there on the South Rim some 5000 feet above the river, having sent three men partway down into the Canyon, they still could not believe—literally—how big the Canyon is.
If you've been there, you probably know the feeling...