Perched on an island’s edge in Georgian Bay, Ontario, the Grotto Sauna is a feat of old- world craftsmanship and new-world sustainability made possible by cutting-edge software and fabrication technology. It is a sculpted space, a sensual experience, and a sophisticated exercise in building science. Inspired by an Italian grotto—a secret water- filled cave concealed within unsuspecting rock formations—our design pays homage to the rugged extremities of the northern Canadian landscape. To the extent that the objective was to tell an architectural story of escape and refuge, we conceived of a design that marries contrasts to transport visitors into an otherworldly sanctum. A simple but dignified exterior built from charred cedar prepared using the traditional Japanese Shou Sugi Ban method conveys a weathered appearance; it’s as if the building has been hidden in plain sight for centuries. By contrast, the warm, curved interior simultaneously emulates Lake Huron’s waves and mirrors the Precambrian shield—a soft, undulating rock surface that has been worn over billions of years. The luminous glow of the interior wood enhances the sunkissed sauna experience, while the pre-aged exterior veils the intimacy of that experience from the world. Importantly, the Grotto is a highly sustainable structure that established a successful methodology for building ambitious architecture in a remote and pristine region. Prefabricating the sauna components off-site minimized the effects of construction on the environment. Site impact was further minimized by a 3D scan of the landscape, which enabled optimal positioning of the structure in the rock. We collaborated directly with fabricators to develop new prototyping and milling methods and with software engineers to write novel software patches for toolpaths. As such, fabricators were able to override the automated limitations of the CNC machinery and use it as a sculpting tool to achieve the aesthetic vision and maximize the available wood.