There’s a mystery hiding deep beneath our feet. If you dig down past the tree roots and house foundations, past the water table and the fossilized bones, through layers of rock and ore, you’ll eventually reach a boundary. Here, the Earth’s crust — the rock we live our lives on — transitions into the denser rock of the Earth’s mantle. This boundary is the Mohorovičić discontinuity, or “Moho” for short. And no one knows for sure what this boundary looks like, or what lies there. Hear more in the Unexplainable podcast episode “Journey Toward the Center of the Earth” with Byrd Pinkerton.