The Lost World at the Bottom of the Deepest Hole on Earth Drop the Empire State Building into the ground, and it would vanish without a trace. That's the scale of the Xiaozhai Tiankeng — a 662-meter wound in the limestone karst of Chongqing, China, the deepest sinkhole on the planet. It wasn't dug. It collapsed. Hundreds of thousands of years ago, the roof of an enormous underground cavern gave way, and the earth simply fell in on itself — twice. The result is a double-nested abyss: an upper bowl plunging 320 meters to a sloping terrace, which then drops another 342 meters into the pit. Stand at the rim and you're staring into a geological Russian doll. But the part that sounds like fiction: at the bottom, there's a forest. A primeval one. The sinkhole's sheer vertical cliffs act like the walls of a thermos — blocking cold winds, trapping humidity — while an underground river runs eight kilometers beneath the floor, breathing moisture up into the chasm. The result is a warm, eerily stable microclimate: a natural greenhouse sealed off from the world above. Inside it, more than 1,280 documented plant species thrive. Towering trees stretch dozens of meters toward the distant rim. Among them grow living fossils — the dove tree, the Chinese yew, wild ginkgoes — ancient relict species that survived the ice ages here while their cousins outside perished. Rare ferns and thick moss carpets cover the limestone floor. And it isn't just plants. Clouded leopards prowl the understory. Leopard cats, Asian black bears, and masked palm civets move through the green dark below, while swallows nest in cities along the rock faces. A whole ecosystem — predators, prey, and thousand-year-old trees — living at the bottom of a hole, in a climate of its own making, that almost no one will ever see. The planet keeps its best secrets underground.