Brunkeberg Tunnel

This guide to Brunkeberg Tunnel in Stockholm opens with the odd acoustic trick of the passage itself, before tracing back to Swedish army captain Knut Lindmark, who dreamed up a toll tunnel through the ridge in the 1880s and solved its collapsing gravel with a refrigeration machine borrowed from the frozen-meat trade. It follows the tunnel's opening in 1886, its financial failure, and the free boulevard that made it obsolete. Try saying something quietly partway through and listen for what comes back.