My world has a semi-nomadic people that inhabit a vast sandy desert. Underneath this desert lie the ruins of an enormous metropolis -- a city that was abandoned tens of thousands of years ago.
In the past fifteen to twenty centuries something has changed in the region which has resulted in the slow uncovering of these long-lost buildings. This process could have been going on for longer, but only in the past few centuries have the ruins actually started to surface.
What is the slow, gradual change that started this? My first thought was a shifting of the direction of the prevailing winds, causing sand to be blown away instead of deposited, but I don't know how realistic this is.
edit: The idea I have in my head is of a once-fertile region that is absolutely crammed with ruins. This city/metropolis could once have had the size and density of Paris or Berlin, for example. This city was abandoned as its people suffered some mysterious mass extinction, but its buildings and their interiors remain mostly intact, owing to the great architectural prowess of its people. The region is now covered in huge amounts of sand and has become a typical sand desert, so that nothing alludes to its former grandeur. Only recently (3~4 centuries) have the tops of the tallest of these buildings (e.g., monuments or other, rare, buildings taller than, say, 80m.) begun to become visible features in the sand.
It is key that none of the peoples who have lived near the region, even for centuries, can recall it being anything other than a desert. Ideally, the uncovering would be plausible as a natural, slow event, but it doesn't have to be scientifically rigorous.
I like the answer below about the flooding of Sarnath, because it explains how the city vanished (I can tie that in with the mass extinction event), how the desert was formed by the settling of salt and sand, and how the sand blows away now the sea is no longer there. The only thing I would then like to know, is what causes the sea to dry up / retreat.