The Sinks are a series of cracks and crevices at the back of the cavern where the water of the Popo Agie River flows in and then reappears at the Rise of the Sinks, a large calm pool a quarter mile down canyon. They are eroded into the soft white Madison Limestone formation, but no spelunkers have explored very far into them because the cracks narrow into very small log- and rock-choked passages. Their exact age is unknown, though they are likely an Ice-Age feature thousands of years old, formed after glaciers carved the canyon, exposed the soluble limestone, and melting ice sent billions of gallons of water through it to create underground passages. For many years it was uncertain whether the water entering the Sinks was the same water emerging at the Rise, but dye tests proved the connection and also revealed that the water takes over two hours to reappear and that more water flows out at the Rise than goes in at the Sinks, leaving the water’s path during that time a mystery.