Look closely, and you’ll see lots of sphagnum moss and other bog plants coating the ground. Bogs form when glaciers dig potholes that fill with rainwater. Clay deposits leftover by the glaciers often line these potholes and keep them watertight. These undrained puddles provide prime conditions for growing sphagnum and duckweed, which actually help filter the water and keep it so acidic that it’s exceptionally clean (despite what the dank odor suggests).