It's one of the great mysteries of the ancient world. Mt. Baigong in the Qinghai Province of China is crowned by a strange pyramid-like structure, full of caves that contain perfectly symmetrical, time-eaten metal pipes. They're inserted deep into the rock, and seem to feed into a nearby lake.
Could it be . . . aliens? According to Atlas Obscura, that's a common belief:
For the rest of the story: http://io9.com/who-built-these-ancient-metal-pipes-deep-inside-a-mount-1258396051The rusty tubes, ranging from needle-size to 16 inches in diameter, reach from deep inside the mountain to a saltwater lake 260 feet away. Many of the hollow pipes are uniform in size and seem to be placed purposefully. The ancient objects are embedded deep enough into the mountain wall and floor to preclude modern human handling. The inhospitable environment surrounding the mountain sees only the occasional nomad. Unless these wanderers developed secret advanced metallurgy skills, the pipes were not formed by human hands.