Sand dunes are dynamic, always changing.

They occur wherever there’s sand, even thousands of miles from the sea. Even on the shores of freshwater lakes.

And the largest freshwater dunes in the world, more than 400 feet high, are in Michigan, on the shores of the Great Lakes.

They’re made of sand formed during the last Ice Age.

Continental glaciers ground up rock as they moved, and deposited sand in the basins that would become the Great Lakes.

Dunes form here, like everywhere, when wind picks up sand and carries it across the land -- until it meets an obstacle, like a rock or shrub.

That slows the wind and causes it to drop its sand, which piles up around the object.

The sand pile grows and becomes its own sand-catcher. More sand is deposited and, over centuries, a dune rises.

The Great Lakes sand dunes have been there at least that long. Though within the last few decades they’ve come under threat.

Sand mining carts them away in truckloads. New buildings block wind that could replenish them. Off-road vehicle traffic cuts some dunes nearly in half.

But state leaders, conservation groups, and volunteers have teamed up to save the dunes.

They’ve declared large areas protected. Limited mining and development. Reestablished native dune plants. Built boardwalks and trails to reduce erosion.

Together, they may be able to preserve the Great Lakes dunes for centuries, and visitors, to come.