Alexander H. Sibley's Silver Islet Mining Company took over the site in 1870 and immediately faced a fundamental problem: there was nowhere to stand. The islet was too small for equipment, too low for storm protection, and Lake Superior's November gales could sweep it clean. The solution was audacious. Workers built wooden breakwaters around the rock and dumped crushed stone into the enclosed space, expanding the islet to over ten times its original size. A small mining town grew on the mainland shore nearby, with houses for the miners and their families, a general store, and the infrastructure needed to keep an underwater mine running. The operation required constant pumping to hold back Lake Superior, which pressed in from all sides, patient and relentless.

For over a decade, Silver Islet produced astonishing quantities of high-grade silver. A second vein discovered in 1878 extended the mine's life, but by 1883 the richest deposits were exhausted and silver prices had dropped. The mine's survival depended on steam-powered pumps that ran day and night, keeping Lake Superior out of shafts that had reached nearly 400 metres deep. Those pumps burned coal. In the autumn of 1883, the season's final coal shipment failed to arrive before the shipping lanes closed for winter. Without fuel, the pumps stopped. Lake Superior, held at bay for sixteen years, flooded the mine shafts in early 1884. No one ever pumped them dry again. Attempts to reopen the mine in 1919 and again in the 1970s, when crews tried reprocessing the mine tailings, came to nothing.

Silver Islet's contribution to mining extended beyond the silver itself. The mine's superintendent, W. B. Frue, developed a new method for extracting metal from low-grade ore using vibrating tables called vanners. The Frue Vanner, first installed at the stamp mill beside Frue's Brook on the mainland, became the first commercially successful ore separation system of its kind. Modern versions of the technology remain in use today. The islet also shaped the lives of those who grew up in its shadow. Julian Gifford Cross, born at Silver Islet in 1888, just four years after the mine closed, went on to found the Steep Rock Iron Mine in Atikokan, Ontario, one of the major iron ore producers in mid-twentieth-century Canada.

The community that once housed silver miners now lives a quieter life. The original houses have been converted into private summer cottages, their wood frames a century and a half old. The general store has been restored and operates as a seasonal tea room, serving light meals alongside knickknacks and basic supplies. Sleeping Giant Provincial Park, which occupies most of the Sibley Peninsula, maintains an exhibit about the mine in its visitor centre. The islet itself still breaks the surface of Lake Superior, smaller now than during its mining years, the breakwaters long since surrendered to the waves. Below the waterline, 384 metres of flooded shafts remain, filled with the cold, clear water of the largest lake on the continent.