Just 40 kilometers northeast of Mexico City, the ruins of an ancient metropolis rise from the Valley of Mexico like a cosmic mystery waiting to be solved. Welcome to Teotihuacán — the "Place Where Gods Were Born" — a city so ancient and enigmatic, even the mighty Aztecs didn’t know who built it.

High in California’s Eastern Sierra, just off Highway 395, lies a body of water so strange it might as well be from another planet. Mono Lake is a landlocked saltwater basin that’s more than 760,000 years old — one of the oldest lakes in North America. But what truly makes it a hidden wonder is what rises from its waters: towering, ghostly formations called tufa.