Horicon Marsh is located about 16 miles southwest of Fond du Lac. A flooded  marsh is low lying land easily flooded, and a place that remains waterlogged at all times.

The northern two thirds of the marsh, about 22,000 acres, is a National Wildlife Refuge managed by the US Fish and Wildlife Service (FWS). The southern one-third, about 11,000 acres, is a State Wildlife Area managed by the Wisconsin Department of Natural Resources (DNR). It is one of the largest intact freshwater wetlands in the US.

I am not normally a prone to visit environmental wonders, but I confess this "exploration" was fun and even exciting. For me the visit turned out to be an adventure.

The Department of Natural Resources (DNR) describes the Horicon Marsh:

"Horicon Marsh is a shallow, peat-filled lake bed scoured out of limestone by the Green Bay lobe of the massive Wisconsin glacier. The glacier entered this area about 70,000 years ago and receded about 12,000 years ago. The same layer of rock that forms the gentle hills to the east of the marsh extends 500 miles to the east and is the same rock layer over which the Niagara River plunges at Niagara Falls. This Niagara Escarpment bordering the marsh, commonly referred to as 'The Ledge' extends for 230 miles in the state of Wisconsin alone. The marsh itself is approximately 14 miles long and ranges from 3-5 miles in width … (It) is a critical rest stop for thousands of migrating ducks and Canada geese … (It is also a) critical habitat for over 300 species of birds as well as muskrats, red foxes, turtles, frogs, bats, dragonflies, fish and much more."

This Green Bay Lobe of the Wisconsin Glacier was powerful. DNR has said:

"The Green Bay lobe encroached upon eastern Wisconsin where it carved out Green Bay. As it moved inland, it etched out the Lake Winnebago Basin and to the south, the Horicon Marsh and Rock River Basin. It reached as far south as the Madison area before it began to retreat due to warming global climates."

Wikipedia has said:

"During the glacier's retreat, a moraine was created, forming a natural dam holding back the waters from the melting glacier and forming Glacial Lake Horicon. The Rock River slowly eroded the moraine, and the lake drained. As the levels of silt, clay and peat accumulated in the former lake's basin, the Horicon Marsh was formed. Occasionally there have even been crop circles that appeared unexpectedly in several areas."

It should be noted that for a while the area was considered useless. Don't tell that to all the wildlife that lives there!