Over millions of years, subterranean streams dissolved the limestone rock to form this spectacular cave system in Nevada's Great Basin National Park. During Prohibition it was used as a secret speakeasy but now the attraction is the thousands of delicate columns, hollow stalactites (known as soda straws) and needle-like crystals. Look out for "cave popcorn", formed from small lumps of calcite. It looks surprisingly like the real thing. It's recommended that you book a ranger-led cave tour in advance as they regularly sell out.