Sado Gold Mine is the island's heavyweight sight: a former gold and silver mine with tunnels, exhibits, and the split mountain of Doyu no Warito marking the old workings. UNESCO lists the wider Sado Island Gold Mines for their Edo-period mining systems, and the visitor tunnels make that history physical rather than abstract.
The standard routes take you through older hand-dug sections and later industrial galleries. Bring a layer even in warm weather; the tunnels are cool, damp, and a little theatrical, which is part of the appeal.