On Douglas Island, reached by crossing the Juneau-Douglas Bridge, the Treadwell Mine was once one of the largest gold mines in the world — a complex of four interconnected mines that in the 1890s employed over 1,000 men and ran a town's worth of infrastructure. A catastrophic cave-in in 1917 destroyed much of it; what remained was left to the temperate rainforest, and the ruins make for one of the more atmospheric walks in Southeast Alaska.
A trail leads through old growth spruce past remnants of the powerhouse, the mill, and the Alaska-Treadwell Gymnasium — used by miners and now just a concrete shell. The forest has swallowed much of it, making the whole area feel genuinely haunted. The walk is about 1.5 miles one way from the parking area off Sandy Beach Road on Douglas Island; the terrain is easy and the beach at Sandy Beach Recreation Area is a good addition.