High above Lake Superior and more than a mile inland, the Devil's Kettle waterfall on the Brule River has enchanted onlookers and stoked the curiosity of scientists for decades. The most visited attraction at Judge C.R. Magney State Park near Grand Marais, it has also been the most puzzling. Above the falls, the river splits in two at an outcropping of rhyolite—volcanic rock as hard as granite. The east side of the river plummets 50 feet into a pool, in typical waterfall fashion. But on the west side, the water plunges into a cavernous hole in the rock and vanishes, leaving observers wondering: Where does all that water go?

Over the years, curious onlookers would sometimes toss a stick or another buoyant item into the swirling waters of the pothole to see if it would resurface downstream. Nothing ever did.

Without seeing an obvious resurgence of water, many people have speculated that the water followed an alternate underground path to Lake Superior. Geologists said that wasn't likely. Underground waterways form in softer rock such as limestone, but the geology of the North Shore is anything but soft. Tunnels, or lava tubes, do not form in rhyolite or in the volcanic basalt far beneath the riverbed.

DNR springshed mapping hydrologist Jeff Green and other scientists have long thought that the water that enters Devil's Kettle didn't divert through a hidden channel to the lake, but rather resurfaced in the river downstream. To test this theory, Green asked the DNR's water monitoring and surveys unit to measure the volume of water flowing above and below Devil's Kettle using stream gauging equipment. By comparing the amount of water flowing above the waterfall with the amount of water flowing below the falls, hydrologists could determine if there was a loss of water somewhere between the locations.

In late fall 2016, hydrologists Heather Emerson and Jon Libbey measured water flow above Devil's Kettle at 123 cubic feet per second. Several hundred feet below the waterfall, the water was flowing at 121 cubic feet per second. "In the world of stream gauging, those two numbers are essentially the same and are within the tolerances of the equipment," Green explains. "The readings show no loss of water below the kettle, so it confirms the water is resurging in the stream below it."

Green and Calvin Alexander, a colleague at the University of Minnesota, plan to conduct a dye trace to show where the water resurfaces. In the fall of 2017, during low-water flow, they will pour a vegetable-based dye into the pothole. This fluorescent, biodegradable dye is visible at 10 parts per billion, so the hydrologists will use only a few quarts.

As far as the mysteriously disappearing items, water force and fluid dynamics offer an explanation. Alexander explains, "The plunge pool below the kettle is an unbelievably powerful system of recirculating currents, capable of disintegrating material and holding it under water until it resurfaces at some point downstream." Unlike larger objects, the dye molecules won't be held in Devil's Kettle.