Marquette is most well-known for his exploration of the Mississippi River, which covered approximately 1,700 miles from the junction of the Wisconsin River with the Mississippi down to the Arkansas River. Marquette traveled with Louis Jolliet and together, not only did they find the river, they traveled through what would one day become Chicago.
Marquette devoted much of his travel time to documentation of the things the travelers saw. He was one of the first Europeans to see and describe much of the wildlife and fauna in the area. They found numerous Native American villages along or near the river and learned much from them, like the fact that the river continued to the gulf.