In this modern day and age, it is hard to imagine the world's biggest city in the world melting into nothing more than a few piles of dirt. Merv, also known as Mary, which is in the middle of the deserts of Turkmenistan, was once the biggest city in the world, and as construction was mostly in mud-brick, most of what it was is now very flat, almost as if a giant raindrop fell and reduced it to sludge. Walking through it meant lots of the silty brownness collected on our shoes, and that has been happening for so long, it's a wonder anything still remains at all.
A major oasis-city that was part of the chain of centers of the Silk Road, Merv does not really have any modern inhabitants, although pilgrimage sites and religious monuments (Muslim and pre-Muslim) are scattered within the walls. Alexander the Great visited, and it was home to Zoroastrians, Buddhists, and many other religions.
I find it an amazing place because hardly anyone's heard of it. It's incredibly isolated, and visitors are quite rare. It is abandoned, yet majestic. Important, yet forgotten. A huge area left wild, with rich history and, of course, a really difficult place to get to.