This week the Flannelcasters talk about the Kola Superdeep Borehole - the deepest borehole ever drilled by humans, extended nearly 12 km below the surface of the Earth.
Links:
Ask Smithsonian: What’s the Deepest Hole Ever Dug?
The deepest hole we have ever dug
How Deep Is the Deepest Hole in the World?
Underground unearthly: What the world's deepest well told about