What once completely covered the area that is now Michigan?

The area that is now Michigan was once completely covered by oceans, specifically the prehistoric warm shallow seas that existed during the Paleozoic era, around 350 million years ago. These ancient seas were part of a larger marine environment that extended over much of the central North American continent, and sedimentary deposits from this period are now found in the state. The geological formations created by these oceans include limestone, shale, and other sedimentary rocks that hold fossilized remains of marine life, illustrating the rich biodiversity that thrived in these ancient waters.

The other options do not apply as directly. While forests have indeed been a significant part of Michigan's landscape, they developed much later in geological history after the oceans receded. The state does not have a history of being covered by mountains or deserts in the same way it was covered by oceans, as the landscape evolved due to tectonic shifts, glacial movements, and changing climates over millions of years. Hence, the presence of ancient oceans is a defining feature in understanding the geological history and development of Michigan's landscape.