We've sent probes billions of miles into space, yet when we tried to dig straight down into our own planet, the Earth fought back and eventually shut us out completely. We know more about the surface of the moon than the rock a few miles beneath our boots.

This episode dives into the Kola Superdeep Borehole, officially SG-3, still the deepest human-made hole on Earth. It's a story of pure scientific ambition colliding with extreme geology, a Soviet-era attempt to pierce the crust that ended in disaster but rewrote our understanding of the ground beneath our feet.

• Why the ancient Baltic Shield offered a rare 'window' into the oldest, hardest rock, with a target of 15,000 meters
• How a year-long pause to celebrate hitting 12,000 meters let the rock clamp down and snap off five kilometers of drill string
• The heat that ...