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Cueva de los Cristales in Mexico, also known as the Cave of the Crystals, is one of the most otherworldly places on Earth. Hidden nearly 300 meters underground, it contains giant selenite crystals, some as long as 11 meters (36 feet) and weighing up to 55 tons. The cave environment is brutal: temperatures can reach 58°C (136°F) with nearly 100% humidity, conditions that make it deadly for humans to explore without protective gear. Scientists can only stay inside for a few minutes at a time. The crystals formed over half a million years as mineral-rich water slowly cooled, allowing molecules to align into perfect, massive structures. Walking inside is like stepping into a landscape from science fiction—a shimmering forest of crystal giants buried deep in the Earth.