Some undiscovered ones may extend far deeper into the Earth than the record of 1.36 miles.
Some of the oddest sites on our planet are below the surface.
See amazing footage from a drone fly-through of the Hang Son Doong cave in Vietnam, the largest in the world.
The microorganisms can survive underground despite the extreme dearth of light and nutrients.
A newly discovered species of reef coral hangs from the ceiling of undersea caves like a colony of petrified bats.
Scientists have found a stalagmite in an Oregon cave that tells the story of thousands of winters in the Pacific Northwest.
In the gloom of the caves and gorges of China, a plant has been discovered thriving in the dark.
A daring, death-defying race to find the planet's deepest hole is chronicled in a gripping new book.
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