The hill looks ordinary but is old, really old, maybe older than Earth. No dating method can place it, Earth's crust simply flows around it.
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It must have embedded itself in the crust like a wooden splinter long ago. Mountain ranges have splashed and broken against it like waves.
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Soil samples taken disappear over night, digging sites close over. Sonar shows a chamber in its heart, but all attempts to breach it fail.
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