For most travelers, that means visiting as part of an organized tour, as companies can arrange letters of invitation that make access easier.What you're missing: Exploring Ashgabat, an over-the-top city that gleams with marble and gold monuments. While many statues of former president Saparmurat Niyazov have been toppled in the years since his 2006 death, the capital still holds the Guinness World Record for the highest density of white marble-clad buildings on earth.A day trip away from Ashgabat is Darvaza Crater, a collapsed, flaming gas cavern known as the "Gates of Hell."