The before-mentioned potholes were created starting with sand and gravel caught in eddies or whirlpools. Within the eddies, the sand and gravel were swirled around with such force that they drilled holes straight down into the rock. Larger rocks caught in the spin would polish the shafts smooth. The Glacial Gardens area on the Minnesota shore contains more than 80 of these glacial potholes, the greatest concentration in the world. When the massive flow of water through the Brule outlet and down the St. Croix River ceased, the potholes became exposed to the air. Dirt, vegetation, and rainwater have collected in them, disguising their true depths. Some have been excavated; one, the Bottomless Pit, is wide and deep, the deepest explored pothole in the world. Other unexcavated potholes in the park are even wider, suggesting that they may be deeper as well.