In July last year, when Lake Superior’s surface temperature warmed into the upper 60s on the back of a heat wave, I snorkeled most days along the shoreline between Duluth and Two Harbors. My 9-year-old, Grace, usually joined me. I wore a wetsuit; she refused one. Despite her lean frame, she would swim for an hour before retreating with purple lips to bask upon the shore’s sunbaked slabs.

We cruised the shallows admiring the luster of submerged stones. With the sun refracting through gin-clear Superior, bits of jasper showed a deep red and chunks of basalt the livid blue of a glacier. Specks of sea glass captured sunbeams like gems. We watched largemouth bass nosing rocks for prey, big-headed sculpins the length of my finger, and, once, the lake awash with pea-sized translucent spheres I later learned were tiny crustaceans called "Holopedium."

Near the mouth of the French River, where, in season, droves of salmon anglers hurled lures from shore, specimens of the most flamboyant fish in the lake, genus "Rapala," hung snagged on sodden logs. Several decades ago, a Finn named Hjalmer Mattson worked a fish camp here. In a deepening cove I swam over thick chains and a rusting tub, relics from the camp. It was Grace, though, who found the sunken remains of Mattson’s dock, a square structure of massive timbers and iron fasteners.

Another afternoon I swam from shore, following the sunlight that flickered across the sandy floor like a rising curtain. The seabed grew indistinct, fading with depth to weak lines beneath cerulean haze. One hundred yards out, the bottom snapped back into focus as a steep gray ridge climbed to a point within several feet of the surface. Beyond, the ridge tumbled toward Superior’s indigo depths. Faced with such immensity, my spine went cold. I spun about and hurried back to land. 

Safe Snorkeling on Superior

• Never underestimate Lake Superior. Be safety conscious if you choose to explore the lake from beneath the surface. Beware of cold water, pounding waves, and dangerous currents. Stay near shore or wear a wetsuit and fins, which add a surprising measure of power to your kick.
• Monitor wave conditions and water temperatures at d.umn.edu/buoys, parkpointbeach.org (link is external), and weather.gov/greatlakes/beachhazards (link is external).
• The French River site is accessed by a new wayside on Scenic 61. Leave historic artifacts for posterity. 
• When conditions allow, the popular swimming spot in Canal Park dubbed Uncle Harvey’s Mausoleum makes for an exciting passage. Perched on a foundation of concrete, iron, and old-growth timber, the wave-battered ruin rising from the lake near the Aerial Lift Bridge was once a gravel hopper.

The Greatest Lake

• Earth’s largest lake in terms of surface area, Lake Superior astoundingly contains 10 percent of the planet’s freshwater and half the total volume of the Great Lakes. National Geographic magazine recently called the Great Lakes North America’s most precious resource.
• Due to its relatively sparsely populated perimeter, Superior has missed out on much of the pollution plaguing the other Great Lakes, but it is experiencing the highest rate of warming of the lakes. This warming, evidenced by the recent emergence of harmful blue-green cyanobacteria blooms, is a threat to its beleaguered cold-water fishery.