Discovered by chance in 2000, adjoining Mexico's Naica silver mine, the Cave of Crystals boasts the world's largest natural crystals, some over 10m in length. At around 50°C with almost 100% humidity, visits without ice cooling suits and masks are confined to ten minutes, to stop breath from condensing in the lungs. Meanwhile, lucky well equipped scientists can enjoy every 20 min the cold room, at just 38°C. They're working in NASA's study led by Penelope Boston, about prehistoric microbes found in the cave.